Detect package better for PRs that don't touch default.nix

When I submit a PR for a bundlerApp package where all I did was modify the Gemfile.lock and gemset.nix files, ofborg doesn't detect the package and therefore doesn't label my PR correctly (e.g. it's missing 8.has: package (update) and 11.by: package-maintainer).

Example PR: https://github.com/NixOS/nixpkgs/pull/66251

I'm not actually sure what logic ofborg uses to detect the packages modified by the PR, so I don't know what the best approach for teaching it about this scenario. One simple possibility would be if the PR title or any commit in the PR follows the format "attrname: …" then check that attribute and see if anything in it was modified by the PR. That simple check would handle the above PR, because the cocoapods-beta attribute is a derivation that references both Gemfile.lock and gemset.nix (and in fact its version attribute is also changed since that comes from gemset.nix).

Start by tracing ofborg's package-detection logic for pull requests and compare it with example PR #66251. Inspect how changes to Gemfile.lock and gemset.nix are associated with the cocoapods-beta attribute, then verify that the PR receives the package update and package-maintainer labels when detection succeeds.