Allow subdirectories in cache path

[stephane-caron]

This proposal follows up to 3b7a70d, which was carried out when adding #166.

• Is your feature request related to a problem?

Yes: 1b786e3

• Describe (concisely) the solution you'd like.

When a package name is used in a URDF file, to be loaded successfully by e.g. PyBullet, there should be a directory with the same name up in the directory tree.

Parsing subdirectories in a repository's cache_path could be a way to handle this.