Add a Bazel build file for mlir/python.

This BUILD file:
* generates machine-generated Python files using tblgen, and
* exports both generated and handwritten Python files via filegroup() rules.

This allows downstream users to use Bazel to build Python wheels that incorporate the MLIR Python bindings.

Reviewed By: GMNGeoffrey

Differential Revision: https://reviews.llvm.org/D112844