The `ModuleManager` uses `FileEntry` objects to uniquely identify module files. This requires first consulting the `FileManager` (and therefore the file system) when loading PCM files. This is problematic, as this may load a different PCM file to what's already in the `InMemoryModuleCache` and fail the size and mtime checks.

This PR changes things so that module files are identified by their file system path. This removes the need of knowing the file entry at the start and allows us to fix the race. The downside is that we no longer get the `FileManager` inode-based uniquing, meaning symlinks in the module cache no longer work. I think this is fine, since Clang itself never creates symlinks in that directory. Moreover, we already had to work around filesystems recycling inode numbers, so this actually seems like a win to me (and resolves a long-standing FIXME-like comment).

Note that this change also requires that all places calling into `ModuleManager` from within a single `CompilerInstance` use consistent paths to refer to module files. This might be problematic if there are concurrent Clang processes operating on the same module cache directory but with different spellings - the `IMPORT` records in PCM files will be valid and consistent within single process, but may break uniquing in another process. We might need to do some canonicalization of the module cache paths to avoid this issue.