Rethink package API

[gkjohnson]

Providing a string to replace package:// is technically incorrect by ROS standards. An option has been added to to provide a package map which could replace the currently functionality entirely. The URDF file should also not be loaded relative to the package path, either.

[gkjohnson]

The urdf file is no longer loaded relative the provided package path.

The remaining confusing bit is that when just a string is provided both loaders will check if the provided path ends with the package name, in which case it will dedupe the directory. This logic may be removed so it doesn't have to be explained in documentation.