Correctly handle @loader_path and @rpath dependencies

[ThomasDebrunner]

macpack did not handle @loader_path and @rpath dependencies. I added the following functionality:

• For every library, find all the rpath in the dependency by issuing and parsing otool -l command on the library.
• If the library has a dependency on a @rpath library, try to find it in any of the libraries rpath.
• If the library has a dependency on a @loader_path library, try to find it in the directory of its requester.

To make it all more generic, I renamed the the symlinks list to referred_as list, which just contains the list of all the strings under which a specific library is referred to as. This can include absolute paths to libraries, paths to symlinks or strings with @loader_path or @rpath. This list is the only thing the patcher then looks at, as it is all the reference strings it must change.

The path member of dependency always contains the resolved, absolute path to the actual library.

[chearon]

Hey thanks for this pull request! It's been a long time since I've looked at this code so I've been putting it off but I do hope to take a look at this soon. I actually didn't even consider that it would be possible to handle @loader_path and @rpath, but this will be great to have.