[8.8.1] Look up directory input children in the input metadata cache again (#31178) <!-- Thank you for contributing to Bazel! Please read the contribution guidelines: https://bazel.build/contribute --> ### Description The 8.x adaptation in #30353 made `DirectoryTreeBuilder` bypass the per-build `SingleBuildFileCache`, which means that every each Merkle tree computation now stat'ed and looked up the digest of every file in the directory again, possibly recomputing the digest if the digest cache wasn't sufficiently large. Builds with large source directories shared by many actions became noticeably slower in 8.8.0 compared to 8.7.0. `DirectoryTreeBuilder` now consults the input metadata provider first and only falls back to digesting the file directly when the lookup fails, which is the case for repos that are only available in the remote repo contents cache's overlay file system. `master` is not affected as `MerkleTreeComputer` caches the subtree of a source directory keyed by its metadata. ### Motivation <!-- Why is this change important? Does it fix a specific bug or add a new feature? If this PR fixes an existing issue, please link it here (e.g. "Fixes #1234"). --> ### Build API Changes <!-- Does this PR affect the Build API? (e.g. Starlark API, providers, command-line flags, native rules) If yes, please answer the following: 1. Has this been discussed in a design doc or issue? (Please link it) 2. Is the change backward compatible? 3. If it's a breaking change, what is the migration plan? --> No ### Checklist - [x] I have added tests for the new use cases (if any). - [ ] I have updated the documentation (if applicable). ### Release Notes <!-- If this is a new feature, please add 'RELNOTES[NEW]: <description>' here. If this is a breaking change, please add 'RELNOTES[INC]: <reason>' here. If this change should be mentioned in release notes, please add 'RELNOTES: <reason>' here. --> RELNOTES: Fixed a regression that caused source directories to be slower to digest for remote execution and cache lookups.
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