ci/package_checks: Add a check for installed licenses

[silkeh]

Summary

Add a check that emits a warning when a package does not include license files.

See #7294

Test Plan

$ common/CI/package_checks.py packages/py/python3/pspec_x86_64.xml 
Checking files: packages/py/python3/pspec_x86_64.xml
Found 1 result(s), 1 warnings and 0 error(s)
WRN packages/py/python3/pspec_x86_64.xml:1: Package is missing a license in /usr/share/licenses/
$ common/CI/package_checks.py packages/s/snapd/pspec_x86_64.xml 
Checking files: packages/s/snapd/pspec_x86_64.xml
Found 0 result(s), 0 warnings and 0 error(s)

Checklist

• Package was built and tested against unstable
• This change could gainfully be listed in the weekly sync notes once merged
• I agree to license this contribution and all my previous contributions under the licensing terms in LICENSE.md and have the power and authority to grant those licenses.

[EbonJaeger]

Unfortunately, it is not quite that simple. Python packages usually install license files to /usr/lib/python3.12/site-packages/${project}-${version}.dist-info/licenses/. Ruby gemfile projects I think also install licenses to the gem directory somewhere. There may be other edge cases that I don't know about.

[silkeh]

Unfortunately, it is not quite that simple. Python packages usually install license files to /usr/lib/python3.12/site-packages/${project}-${version}.dist-info/licenses/. Ruby gemfile projects I think also install licenses to the gem directory somewhere. There may be other edge cases that I don't know about.

I've changed the check to use a list of patterns which can easily be extended. That should make it possible to cover all the situations. It's also a warning, so it shouldn't block PRs when no licenses can be found.

[EbonJaeger]

Oops, I didn't see that this had been updated. LGTM, thanks!