fix 43 occurences of eg.:
> ValueError: Mesh package://example-robot-data/robots/ur_description/meshes/ur5/collision/base.stl could not be found.
Following 0c0b3af793, pinocchio has
hpp-fcl support, so example-robot-data try to load meshes.
But for `package://example-robot-data/robots/…/mesh.stl` paths to be
resolved by the mesh loader, there must be a
`example-robot-data/robots` folder.