feat: add support for excludedDirs into FileSymbolLoader

[simPod]

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Required for composer-unused/composer-unused#431

[icanhazstring]

@simPod could you check the failed CI checks?
After that looks fine. We could merge 🚀

[simPod]

Bam!

Yup, I usually don't run stuff locally and rely on CI but since I have not contributed here yet it did not run because of github's default setting. Should be fine now.