Refactor CLI code

[trishankatdatadog]

What does this PR do?

Refactors CLI code and fixes integration test.

Motivation

Originally wanted to deprecate usage of simple indices in the new downloader, by reading off versions of integrations directly from TUF metadata instead of depending on Python simple indices.

Unfortunately the reference implementation of TUF does not yet support doing this in an elegant and future-proof way. This is because it does not yet allow for the easy and foolproof verification of delegated targets metadata.

Therefore, I have refactored the CLI code to be simpler (by moving code to the more appropriate download module instead). I also fixed the integration test to pick up all known wheels (bug in regex).

Additional Notes

N/A.

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #3317 into master will decrease coverage by 5.48%.
The diff coverage is 82.92%.

@@            Coverage Diff             @@
##           master    #3317      +/-   ##
==========================================
- Coverage    85.9%   80.42%   -5.49%     
==========================================
  Files         745        8     -737     
  Lines       38692      235   -38457     
  Branches     4563       22    -4541     
==========================================
- Hits        33239      189   -33050     
+ Misses       4179       36    -4143     
+ Partials     1274       10    -1264

[zippolyte]

LGTM, changing label to Added since it moves some code around