Export targets in vtkm namespace

Currently, when VTK-m exports targets, it exports them all in the global namespace. This is bad style as there are potential conflicts with the exports from other CMake libraries.

The VTK-m CMake configuration should export all of its targets in the vtkm:: namespace. For existing targets that are expected by external projects, the old target names should still exist (until VTK-m 2.0), but using them should somehow print a deprecation warning.