Allow build deps as long as at least one setup file is in sources list.

[Walter-Gates-Bose]

Resolves 2076.

Passing --build-deps-for or --all-build-deps should not emit an error as long as at least one file in the sources list is a setup file.

Using pip-compile --all-build-deps requirments.in pyproject.toml gives the error --build-deps-for and --all-build-deps can be used only with the setup.py, setup.cfg and pyproject.toml specs..

According to the help, using multiple source files is valid, so any options that apply to at least one source file should be allowed as long as that option does not cause a conflict.

Contributor checklist

• Included tests for the changes.
• PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• Verified 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).

[webknjaz]

I'm 👎 on this one since it opens up a can of worms (#2076 (comment)) — the implication of such an interface would be that the build deps of everything listed in the requirement inputs would be collected which is simply untrue.

[webknjaz]

cc @apljungquist ^