[bazel]: de-alias pybind11 headers target

[j2kun]

In trying to set up python headers in an out-of-tree bazel MLIR project, I encountered the pybind11_bazel project, and found that the @python_runtime target used here is not defined by it.

Instead, it seems that @python_runtime is an alias used in some projects like Tensorflow (see
https://github.com/tensorflow/tensorflow/blob/322936ffdd96ee59e27d028467fe458859cf3855/third_party/python_runtime/BUILD#L7-L7), where it is aliased to @local_config_python. In fact, @local_config_python is defined by @pybind11_bazel, and so it seems that this layer of indirection no longer serves a purpose, and instead just prevents anyone who doesn't clone Tensorflow's config from using the python bindings here.

This commit updates the dependent targets to their canonical de-aliased equivalents, and I suspect this will not even break any downstream users since the new target is defined in those projects already.

Without this change, running, for example

bazel build @llvm-project//mlir:MLIRBindingsPythonCore

gives the error

no such package '@python_runtime//': The repository '@python_runtime'
could not be resolved: Repository '@python_runtime' is not defined and
referenced by '@llvm-project//mlir:MLIRBindingsPythonCore'

Minimal reproduction in https://github.com/j2kun/test_mlir_bazel_pybind, which, when pointing to a local LLVM repository that has this change (see bazel/import_llvm.bzl in that repository), results in that build succeeding.

Hat tip to Maksim Levental for going on an hours-long investigation with me to figure this out.

[makslevental]

I hereby do solemnly swear that I witnessed all the bazel insanity first-hand and this fix makes sense to me 🤚.