Support bzlmod

This is the very start of making rules_rust being able to load dependencies via bzlmod. Currently the aim is to replace `rules_rust_dependencies` only.

There is one new module, `examples/bzlmod/hello_world`, that depends on the root `rules_rust` module. This can be built by:

```
cd examples/bzlmod/hello_world
bazel build //:hello_world
```

There is currently some strange stuff going on with visibility because I get the following error:

```
While resolving toolchains for target //:hello_world: com.google.devtools.build.lib.packages.BuildFileNotFoundException: no such package '@rules_rust.override//rust': The repository '@rules_rust.override' could not be resolved: Repository '@rules_rust.override' is not visible from repository '@rules_rust.override'
```

A package is not visible to its self?

MODULE.bazel

@@ -0,0 +1,22 @@
+module(
+    name = "rules_rust",
+    version = "0.9.0",
+)
+
+bazel_dep(name = "platforms", version = "0.0.5")
+bazel_dep(name = "rules_cc", version = "0.0.1")
+bazel_dep(name = "bazel_skylib", version = "1.2.0")
+bazel_dep(name = "apple_support", version = "0.13.0")
+
+# TODO: Implement the equivalent of this:
+
+# # process_wrapper needs a low-dependency way to process json.
+# maybe(
+#     http_archive,
+#     name = "rules_rust_tinyjson",
+#     sha256 = "1a8304da9f9370f6a6f9020b7903b044aa9ce3470f300a1fba5bc77c78145a16",
+#     url = "https://crates.io/api/v1/crates/tinyjson/2.3.0/download",
+#     strip_prefix = "tinyjson-2.3.0",
+#     type = "tar.gz",
+#     build_file = "@rules_rust//util/process_wrapper:BUILD.tinyjson.bazel",
+# )

examples/bzlmod/hello_world/.bazelrc

@@ -0,0 +1 @@
+build --experimental_enable_bzlmod

examples/bzlmod/hello_world/BUILD.bazel

@@ -0,0 +1,14 @@
+load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_doc")
+
+package(default_visibility = ["//visibility:public"])
+
+rust_binary(
+    name = "hello_world",
+    srcs = ["src/main.rs"],
+    deps = ["@examples//hello_lib"],
+)
+
+rust_doc(
+    name = "hello_world_doc",
+    crate = ":hello_world",
+)

examples/bzlmod/hello_world/MODULE.bazel

@@ -0,0 +1,8 @@
+module(
+    name = "hello_world",
+    version = "1.0",
+)
+
+bazel_dep(name = "rules_rust", version = "0.9.0")
+
+local_path_override(module_name = "rules_rust", path = "../../..")

examples/bzlmod/hello_world/src/main.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+extern crate hello_lib;
+
+use hello_lib::greeter;
+
+fn main() {
+    let hello = greeter::Greeter::new("Hello");
+    hello.greet("world");
+}