- Fix a typo in the debug output for finding a prebuilt module
- Don't add the prebuilt module path to the dependency tracker
- Ensure we're registering the _forwarding module_'s dependencies in
cached modules, not the prebuilt module's dependencies.
- Check for the existence of a prebuilt module before doing the
is-up-to-date check (which will have failed anyway if it didn't exist).
- Test that forwarding module dependency collection works
rdar://48659199
