refactor(config): fix warnings by 'cargo clippy' for config

[octonawish-akcodes]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

issue #191

2. What is the scope of this PR (e.g. component or file name):

KCLVM/config

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

Before:

abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm/driver$ cargo clippy --fix --allow-dirty
    Checking kclvm-driver v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/driver)
warning: unnecessary `if let` since only the `Ok` variant of the iterator element is used
  --> driver/src/lib.rs:31:5
   |
31 |       for path in paths {
   |       ^           ----- help: try: `paths.flatten()`
   |  _____|
   | |
32 | |         if let Ok(path) = path {
33 | |             matched_files.push(path.to_string_lossy().to_string());
34 | |         }
35 | |     }
   | |_____^
   |
help: ...and remove the `if let` statement in the for loop
  --> driver/src/lib.rs:32:9
   |
32 | /         if let Ok(path) = path {
33 | |             matched_files.push(path.to_string_lossy().to_string());
34 | |         }
   | |_________^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_flatten
   = note: `#[warn(clippy::manual_flatten)]` on by default

warning: you seem to use `.enumerate()` and immediately discard the index
  --> driver/src/lib.rs:64:22
   |
64 |     for (_, file) in k_files.iter().enumerate() {
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_enumerate_index
   = note: `#[warn(clippy::unused_enumerate_index)]` on by default
help: remove the `.enumerate()` call
   |
64 |     for file in k_files.iter() {
   |         ~~~~    ~~~~~~~~~~~~~~

warning: writing `&PathBuf` instead of `&Path` involves a new object where a slice will do
   --> driver/src/lib.rs:207:29
    |
207 | pub fn lookup_kcl_yaml(dir: &PathBuf) -> io::Result<PathBuf> {
    |                             ^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
    = note: `#[warn(clippy::ptr_arg)]` on by default
help: change this to
    |
207 ~ pub fn lookup_kcl_yaml(dir: &Path) -> io::Result<PathBuf> {
208 ~     let mut path = dir.to_path_buf();
    |

warning: `kclvm-driver` (lib) generated 3 warnings
warning: `assert_eq` of unit values detected. This will always succeed
  --> driver/src/tests.rs:79:5
   |
79 | /     assert_eq!(
80 | |         expand_input_files(&input_files).sort(),
81 | |         expected_files.sort()
82 | |     );
   | |_____^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_cmp
   = note: `#[warn(clippy::unit_cmp)]` on by default

warning: `assert_eq` of unit values detected. This will always succeed
   --> driver/src/tests.rs:115:5
    |
115 | /     assert_eq!(
116 | |         expand_input_files(&input_files).sort(),
117 | |         expected_files.sort()
118 | |     );
    | |_____^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_cmp

warning: `kclvm-driver` (lib test) generated 5 warnings (3 duplicates)
    Finished dev [unoptimized + debuginfo] target(s) in 1.74s
warning: the following packages contain code that will be rejected by a future version of Rust: rustc-serialize v0.3.24
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm/driver$ cd .
abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm/driver$ cd ..
abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm$ cd config
abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm/config$ cargo clippy --fix --allow-dirty
    Checking rustc_data_structures v0.0.1
    Checking kclvm-runtime v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/runtime)
   Compiling kclvm-version v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/version)
    Checking rustc_span v0.0.1
    Checking compiler_base_span v0.0.1
    Checking compiler_base_span v0.0.2 (/home/abhi_ubuntu/kcl1/kcl/compiler_base/span)
    Checking compiler_base_error v0.0.10
    Checking kclvm-span v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/span)
    Checking compiler_base_session v0.0.13 (/home/abhi_ubuntu/kcl1/kcl/compiler_base/session)
    Checking kclvm-error v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/error)
    Checking kclvm-ast v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/ast)
    Checking kclvm-config v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/config)
       Fixed config/src/modfile.rs (2 fixes)
       Fixed config/src/path.rs (5 fixes)
       Fixed config/src/cache.rs (1 fix)
warning: the borrowed expression implements the required traits
   --> config/src/path.rs:139:31
    |
139 |                         .join(&sub_path)
    |                               ^^^^^^^^^ help: change this to: `sub_path`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
    = note: `#[warn(clippy::needless_borrows_for_generic_args)]` on by default

warning: `kclvm-config` (lib) generated 1 warning (run `cargo clippy --fix --lib -p kclvm-config` to apply 1 suggestion)
       Fixed config/src/settings.rs (1 fix)
       Fixed config/src/path.rs (8 fixes)
    Finished dev [unoptimized + debuginfo] target(s) in 7.42s
warning: the following packages contain code that will be rejected by a future version of Rust: rustc-serialize v0.3.24
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm/config$ cargo clippy --fix --allow-dirty
    Checking kclvm-config v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/config)
       Fixed config/src/cache.rs (1 fix)
       Fixed config/src/path.rs (5 fixes)
       Fixed config/src/modfile.rs (2 fixes)
warning: the borrowed expression implements the required traits
   --> config/src/path.rs:139:31
    |
139 |                         .join(&sub_path)
    |                               ^^^^^^^^^ help: change this to: `sub_path`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
    = note: `#[warn(clippy::needless_borrows_for_generic_args)]` on by default

       Fixed config/src/settings.rs (1 fix)
       Fixed config/src/path.rs (8 fixes)
warning: `kclvm-config` (lib) generated 1 warning (run `cargo clippy --fix --lib -p kclvm-config` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 2.17s
warning: the following packages contain code that will be rejected by a future version of Rust: rustc-serialize v0.3.24
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`

After:

abhi_ubuntu@MADHUSANJ:~/kcl1/kcl/kclvm/config$ cargo clippy --fix --allow-dirty
    Checking kclvm-config v0.7.4 (/home/abhi_ubuntu/kcl1/kcl/kclvm/config)
    Finished dev [unoptimized + debuginfo] target(s) in 2.37s
warning: the following packages contain code that will be rejected by a future version of Rust: rustc-serialize v0.3.24
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`

[coveralls]

Pull Request Test Coverage Report for Build 7639362436

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.787%

---

Totals
Change from base Build 7638731554:	0.0%
Covered Lines:	2293
Relevant Lines:	2612

---

💛 - Coveralls

[Peefy]

LGTM

[coveralls]

Pull Request Test Coverage Report for Build 7639362439

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-15.4%) to 72.349%

---

Totals
Change from base Build 7638731554:	-15.4%
Covered Lines:	43729
Relevant Lines:	60442

---

💛 - Coveralls