Validate CODEOWNERS file programmatically across path changes

[ShamrockLee]

Issue description

There's currently no automatical ways to validate the paths inside the CODEOWNERS file. When files or directories are renamed, the path patterns may become invalid, but the error would be ignored silently across the PR review, unless the author found it during search/replace or when it is pointed out by a reviewers. It is very likely that no one ever thought about that file, and it doesn't get included in the path-changing PR at all.

I encountered this issue when working on #262648.

Steps to reproduce

1. Make a feature branch inside a Nixpkgs fork.
2. Rename an arbitrary path mentioned inside CODEOWNERS.
3. Update the files other than CODEOWENRS referencing the changed path, to make the build passes.
4. Run nixpkgs-review rev HEAD, and make sure there's no unrelated problems.
5. Make a draft pull request in the comparison page of the base branch and the feature branch.
6. See it passes all the changes. The CODEOWNERS path invalidation silently passes all the test cases.

Technical details

• system: "x86_64-linux"
• host os: Linux 6.1.55, NixOS, 23.05 (Stoat), 23.05.20231007.5a237ae
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.15.2
• nixpkgs: /nix/store/3fdrwsy2yi8rwxqylpd6vi2asraxq1nv-source

[FliegendeWurst]

Done in #336261 by switching to homegrown codeowner solution.