Make makeUsdPath() handle namespace in DAG path

[Daniel 'Baz' Barry]

Please make makeUsdPath() in Maya->USD export facility handle namespaces in DagPaths somehow. The SdfPath in USD doesn't like colons, which are used as namespace separators in Maya. The current implementation returns an empty SdfPath as that's exactly what SdfPath::SdfPath() leaves when presented with an invalid path.

For example:

MDagPath makeDagPath(const char*);
makeUsdPath(makeDagPath("|a|b"), makeDagPath("|a|b|c:d")); // returns empty SdfPath

@aloysbaillet