file_sys: Consolidate separate update directory handling.

[squidbus]

• Adds a utility for iterating contents of a guest directory, applying contents from separate update path as needed.
• Use this utility for guest dirents and loading from sce_module, eliminating the need to special-case separate update directory by just going through the mount point system.

In the future we could use this to handle injecting mount point directories into the entries list as well, if there are mount points within other mount points. For now it does not handle this case.