Add resolve_filepath for url scheme such that package://, model://

[iory]

This PR supports URL scheme such that package://, model://. With this PR, we can load urdf contains following XML tag.

<mesh filename="package://fetch_description/meshes/base_link_collision.STL" />

[mmatl]

So sorry for the delayed reply, just saw this! Nice PR, I'll test it and merge it soon.

Best, Matt

[sthoduka]

btw, I've tested this (at least for package:// URLs) and it works fine. Thanks @iory!

[mcevoyandy]

Hi @mmatl I also wanted this functionality so was looking into why this was failing... there's a few issues I found, but it seems the tests won't pass even on master? the _to_xml function in urdf.py ends up looking for the meshes in the wrong place?