[roll] Roll fuchsia [check-licenses] Refactor directory traversal and testing

This change addresses numerous bugs, performance bottlenecks, and
concurrency risks in the `directory` package. It also completely
overhauls the testing strategy to use fast, isolated unit tests rather
than brittle Golden JSON integration files.

* Symlink Safety: Fixed a critical traversal bug where symlinks
  pointing to directories were incorrectly treated as files. The crawler
  now explicitly resolves symlink targets to avoid infinite loops and
  misattributions.

* Cache Miss Fix: Fixed an issue where the `project.AllProjects` cache
  was being queried with absolute paths instead of relative paths,
  causing a 100% cache miss rate and forcing redundant README parsing.

* Concurrency Safety: Encapsulated the global `AllDirectories` map
  behind a `sync.RWMutex` with thread-safe `AddDirectory()` and
  `GetAllDirectories()` accessors, preventing data races during
  compliance report generation.

* Performance: Optimized `readme.go`'s `readmeFileExists` check to use
  O(1) `os.Stat` calls instead of expensive O(N log N) `os.ReadDir`
  sorting operations.

* Code Cleanup: Deleted duplicate logic blocks in `directory.go`. Moved
  the contents of `init.go` into `config.go` and deleted `init.go`.

* Testing: Purged the `testdata/` directory and `want.json` files.
  Rewrote `directory_test.go` and created `config_test.go` to use
  dynamic `t.TempDir()` scaffolding, aggressively covering symlinks,
  barriers, inheritance, and config merging.

* Documentation: Added a comprehensive `README.md` documenting the
  architecture, with a note regarding the future refactoring of
  `readme.go` logic into the `project` package.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1567053
Original-Revision: 10190589bc6a69bf5d568ef85ad13ddf7dd4ff63
GitOrigin-RevId: 4b43897d65c99da7827d1922a41b351f03ee3f7c
Change-Id: Ibb3c322d2ca12389e341ebf524b47066a79863ad