Update workspace-tools and simplify hasher logic

[ecraig12345]

Update workspace-tools to 0.41.0 and make corresponding API updates.

Remove backfill-hasher's dependency on @rushstack/package-deps-hash (mainly to avoid pulling in @rushstack/node-core-library and all its unneeded deps), and export helpers that are currently duplicated by lage. The logic from getPackageDeps has been copied/modified in a new file getFileHashes to more accurately reflect what it does.

Improve documentation and simplify logic throughout backfill-hasher:

• Document the key and value formats for various internal structures, and ensure the tests use realistic input
• Use for..of instead of .forEach() since .forEach() can cause perf issues in hot paths
• Simplify resolveExternalDependencies and remove unnecessarily confusing resolveInternalDependencies which is actually a trivial operation (both copying what was already done in lage)
• Remove trivial helpers which are better handled inline or via workspace-tools
• Change RepoInfo to internally use PackageInfos format to store info about repo-internal packages, instead of WorkspaceInfos, since the mapping from name to package is more efficient for some usage