CMAKE: fix vtkm devices namespaces
This commit completes the cmake namespace vtkm::
feature and adds:
- Consistency to export every device (before some of them were not being exported).
- Correctly mangle the names for the device so that internally they are
vtkm_
and externally arevtkm::
These changes are already being tested in VTK (MR: vtk!9762 (merged))
Backport: release
Edited by Vicente Bolea