mpi_collectives: Refactor to fix build issues (#15534)

* mpi_collectives: Refactor to fix build issues

After TF commit 5c7f9e31, the mpi_collectives package would no longer build
ops and kernels. This build issue caused mpi_collectives import to fail in
Python with the following error: "NameError: Could not find operator MPISize
in dynamic library mpi_collectives.so".

To fix this issue, add build targets to ensure both ops and kernels are built
Note, also refactored the build targets and directory structure to more
closely match other contrib packages.

* mpi_collectives: Minor BUILD fix

* Minor: Buildifier fix

* mpi_collectives: Correct preprocessor defines

* mpi_collectives: Clearer defines inclusion