Remove extras from user-supplied constraints in backtracking resolver

[thomdixon]

This change ensures extras are removed from user-supplied constraints in the backtracking resolver, fixing #1806 and dependabot/dependabot-core#6550. Credit goes to @deivid-rodriguez for the suggestion.

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[jeffwidman]

Thanks for putting up this PR!

Given the suggestion came from Deivid and copies his patch, you might want to include him as an additional author on the commit?

Is it possible to include a test to ensure no future regression?

[thomdixon]

Given the suggestion came from Deivid and copies his patch, you might want to include him as an additional author on the commit?

Thanks for the suggestion! I amended the commit and force-pushed the update.

Is it possible to include a test to ensure no future regression?

Unfortunately, there's only one unit test that uses the backtracking resolver fixture, and it's not fully fleshed out with the required options, nor an InstallCommand that has the appropriate behavior to actually exercise this code. Further, using e.g., mock.MagicMock proved difficult due to type assertions in the code.

[deivid-rodriguez]

Yeah, I actually noticed that by removing a few related lines and seeing that no tests would fail. Hopefully we can get some guidance on how to proceed, maybe get the fix introduced for now based on our realworld testing until unit tests can be improved.

[thomdixon]

Just an update: I added an integration test for this case in order to prevent regression. Without this PR the test fails and with the PR the test passes.

[atugushev]

LGTM 👍