Add DirectoryPath for nsjail genrule

This adds a new interface DirectoryPath representing directories
specified by dirgroup modules. As directories are not regular files,
DirectoryPath is meant to be incompatible with regular Path.

Bug: 375551969
Test: m nsjail_genrule_test
Change-Id: I55806121a3a222a8b02f1a080f25448d425447b3