Fix order-dependent SRA

What does this MR do and why?

When multiple SBOMs exist (e.g., root pom.xml and subdirectory metrics/pom.xml), the BelongsToProject() classifier incorrectly treated root directory . as having depth 1 instead of 0. This caused it to tie with subdirectories at depth 1, making results dependent on input order.

Solution

Modified countDirs() to return 0 for root directory ., ensuring it's always sorted as least specific (last) regardless of input order.

Manual testing

See gitlab-org/gitlab#577752 (comment 2834687595)

Related issues

DS: Fix java reachability E2E tests (gitlab-org/gitlab#577752) • Orin Naaman • 18.6 Update sbom comparison job to validate multiple... (gitlab-org/security-products/tests/gson-fork!2 - merged) • Orin Naaman • 18.6

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