BaseDirectoryLoader: Make base loader check the fetched artifact

This moves the path mangling in tarball directory loader.

This is the root cause of an issue in the loader git directory's current implementation. [1]

Another commit also adds the exclude vcs flag to avoid hashing metadata vcs folders in the nar hash computation.

Refs. swh/meta#4979 (closed)

[1] swh-loader-git!154 (comment 139545)