Clean up hasher code

[ecraig12345]

While looking at doing a workspace-tools API usage update, I noticed and addressed some oddities in the TargetHasher code:

• Instead of calling getWorkspacesAsync following by a confusingly-named local getPackageInfos method, just call getPackageInfosAsync directly
• Remove the misleadingly-named resolveInternalDependencies helper and replace it with inline Object.keys().filter() (since that's effectively what it did)
• Clarify the code of resolveExternalDependencies and dedupe redundant tests
• Mark all TargetHasher methods/properties that don't appear to be used publicly as private, for clarity for future maintenance. This is technically a breaking change, but I'm not concerned in this case since based on npm stats there don't appear to be other consumers of @lage-run/hasher.