[MLIR] Update Bazel build to finish moving PDL-related transform ops into an extension

https://reviews.llvm.org/D151104 moved PDL-related transform ops into an extension and updated the Bazel build, but one tiny thing fell through the cracks - TransformOpsPyFiles also needs to include the newly introduced `mlir/python/mlir/dialects/_transform_pdl_extension_ops_ext.py`.

Reviewed By: saugustine, bkramer

Differential Revision: https://reviews.llvm.org/D151368