feat: follow directory symlinks during license file search

[Eljees]

Closes #401

Summary

grant's local license-file search now resolves and searches symlinked directories instead of skipping them, so licenses that are only reachable through a directory symlink (the snap layout from #70) are discovered.

Details

• Directories to search are kept in a small queue; a symlink whose target is a directory is queued instead of being handed to the classifier.
• Each directory is resolved with filepath.EvalSymlinks and searched at most once, so symlink cycles (a -> b -> a, or a link back to the scan root) terminate.
• The walk runs against the resolved path: filepath.WalkDir applies lstat semantics to its root as well, so walking a symlinked directory by its link path would not descend into it.
• Matched files are deduplicated by resolved path, so a license reachable both directly and through a symlink is reported once.
• Broken symlinks are still skipped, and skipDirectories names are honored for directory symlinks too.

skipSymlink was replaced by symlinkTarget, which distinguishes "not a symlink" / "symlink to dir" / "broken symlink" instead of collapsing the last two into a single skip.

Tests

TestSearchLicenseFiles_Symlinks updated and extended (go test ./grant/ passes):

• follows symlink to directory (was skips symlink to directory)
• finds license only reachable through a symlinked directory — new; fails without this change
• terminates on symlink cycles — new; a directory that symlinks back to its parent
• existing follows symlink to file and skips broken symlink cases still pass