[bazel]: de-alias pybind11 headers target (#79676)

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.