Allowing relative paths in URDF (#1213)

When parsing a URDF file, the `_source` parameter is set to "urdf file" which breaks relative path resolution downstream. This commit changes the call to `sdf::readDoc` to use `filename` as the `_source` parameter instead.

Signed-off-by: Robert Plante <RobertDPlante@gmail.com>
Co-authored-by: Addisu Z. Taddese <addisu@openrobotics.org>

src/parser.cc

@@ -415,7 +415,7 @@ bool readFileInternal(const std::string &_filename, SDFPtr _sdf,
   {
     URDF2SDF u2g;
     TiXmlDocument doc = u2g.InitModelFile(filename);
-    if (sdf::readDoc(&doc, _sdf, "urdf file", _convert, _errors))
+    if (sdf::readDoc(&doc, _sdf, filename, _convert, _errors))
     {
       sdfdbg << "parse from urdf file [" << _filename << "].\n";
       return true;