tailor should not create targets for unparseable requirements.txt/pyproject.toml/etc.

[stuhood]

As described in #15276, tailor will currently generate targets which cover invalid requirements.txt files (and although not covered there, also pyproject.toml, etc). While improving our support for parsing those file types is definitely beneficial, our utmost priority should be that tailor leaves a repository in a state where a command like peek will not fail.

To that end, the Python tailor implementation should begin doing some lightweight validation of the files consumed by these macros before creating targets for them, and warning for any files that it will skip (with a message indicating how to silence the warning).

[chrisjrn]

Validation for requirements.txt files now work; will add pipenv and poetry next.