[roll] Roll fuchsia [check-licenses] Refactor project package and add tests

This change resolves critical data races, memory leaks, and silent
parsing failures within the `project` package. It also completely
overhauls the testing strategy for the package to use isolated unit
tests rather than integration-level golden files.

* Data Race Fixes: Added a `sync.Mutex` to the `Project` struct to
  protect `RegularFiles` and `LicenseFiles` from concurrent mutations
  by the directory crawler. Added a thread-safe `GetFiles()` method
  and updated `filter.go` to use it.

* Global State Safety: Encapsulated the global maps `AllProjects`,
  `FilteredProjects`, and `DedupedLicenseData` behind `sync.RWMutex`
  locks with thread-safe getters and setters in `config.go`.

* Memory Optimization: Removed the massive string allocations in
  `filter.go`'s deduplication logic. It now uses the cached,
  pre-computed `Hash()` strings as map keys instead of allocating new
  strings for the entire license text.

* SPDX Bug Fixes: Fixed two bugs in `spdx.go`. It now correctly
  defaults to "NOASSERTION" if a project has no licenses, and it no
  longer generates mathematically invalid `()` string expressions if a
  file contains zero data segments.

* Parsing Bug Fix: Updated `NewProject` to use the exported
  `string(file.SingleLicense)` constant, resolving a bug where
  explicitly defined license files failed to parse.

* Code Cleanup: Purged dead `getGitInfo` code and the unused `git`
  and `ctx` variables. Moved `init.go` contents into `config.go`.

* Testing: Purged the `testdata/` directory and rewrote
  `project_test.go` and `spdx_test.go` to dynamically scaffold tests
  using `t.TempDir()`.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1566402
Original-Revision: 7417934a66ba0271c93896dfc5053775e455a0d7
GitOrigin-RevId: bae527f38323be1db122df06776ed05d4953b333
Change-Id: Icda6eb69af19541de0105ad922ef4f9d4fb3759c