feat: lint-file subcommand

[klmcadams]

Added lint-file subcommand to allow people to lint specified file(s). In the command line, you can run reuse lint-file <files>. For example, reuse lint-file src/reuse/download.py src/reuse/_annotate.py. If you only run reuse lint-file with no files listed, then it will lint all files (as if you were just running reuse lint).

Let @SMoraisAnsys or I know if you have any questions or concerns! This PR addresses the suggestions in #903.

Here is a picture of running reuse lint-file in the command line with a file that doesn't exist (dne_file.py), two files that exist and are compliant, and one file that exists but is not compliant (empty_file.py):

• Added a change log entry in changelog.d/<directory>/.
• Added self to copyright blurb of touched files.
• Added self to AUTHORS.rst.
• Wrote tests.
• Documented my changes in docs/man/ or elsewhere.
• My changes do not contradict
  the current specification.
• I agree to license my contribution under the licenses indicated in the
  changed files.

[carmenbianca]

Hi @klmcadams ! Thank you so much for this PR. Unfortunately I do not have time to review it this or next week.

[klmcadams]

@carmenbianca that's okay! Thanks for letting me know

[carmenbianca]

I did a cursory review. There's a number of things that need to change. Because I have time and this feature has some priority for me, I will work on patching this PR up.

Thank you @klmcadams !

[klmcadams]

@carmenbianca Thanks for the review! Let me know if I can help in any way

[carmenbianca]

Rebased on top of main to fix a problem I had (#1067) while testing, and patched this up in one rather big commit.

Because the behaviour of lint-file is very slightly esoteric (as are so many things in reuse…), I will need to set aside some additional time to improve the man page.

Thanks a lot for the template to work on top of @klmcadams ! You had a lot of the right ideas.

If you have time, I would love a code review on this. If not, that is very understandable :) It's taken me a month to respond to you, after all.

edit: Of course the tests fail on CI and not on my computer! I'll need to fix that, too.

[SMoraisAnsys]

LGTM. You just need to remove the "Unused licenses" section from reuse-lint-file.rst.

[carmenbianca]

Perfect, merging! I'll get a release of this out the door some time soon. Thank you @klmcadams and @SMoraisAnsys !