refactor(providers): specify NIXPACKS_PYTHON_PACKAGE_MANAGER

[lmmx]

This PR refactors the Python rewrite package manager choice logic using an enum to make the decision tree explicitly clear, and thus also to allow a particular branch to be specified in config

• Motivated by a wish ([FEAT] Allow user to specify a preferred package manager #1301) to allow for the case where a Python project may have multiple valid lock files, and to override the default preference, e.g. to reverse the default of Poetry.lock > uv.lock

Tasks

• refactors the Python providers module taking care to preserve the exact pre-existing behaviour (so no existing projects will choose a different package manager to install from)
• Tests are added/updated
  • Unit tests to correctly resolve config variable for NIXPACKS_PYTHON_PACKAGE_MANAGER as "poetry", "uv", etc.
• Docs are updated

[madisvain]

Currently, when both requirements.txt and uv.lock exist in a project, the system automatically prioritizes pip (via requirements.txt) because of the fixed priority order in the code. I need to keep requirements.txt in my project for general pip compatibility, but specifically on Railway I'd like to try using uv instead. This PR would allow overriding the default package manager selection through the NIXPACKS_PYTHON_PACKAGE_MANAGER configuration variable, making it possible to choose uv even when requirements.txt is present.

[coffee-cup]

This looks great! ty

[lmmx]

yw, will take a look at the clippy lint failure my bad sorry!

[lmmx]

Reviewable once more 🫡

[coffee-cup]

Just a few more linting failures. You should be able to fix these automatically (or see where they are) with cargo lint-fix locally. Make sure you are using the latest Rust version.

[lmmx]

I just see one, think I got it!

[github-actions]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days

[lmmx]

This pull request is stale because it has been open 10 days with no activity. Remove stale label or comment or this will be closed in 5 days

Ready to review, not stale! @coffee-cup