move to rules_python and remove local_config_python

[junyer]

Although bazelbuild/rules_python#824 isn't formally "done" yet, it seems that pybind11_bazel can move wholesale to rules_python (i.e. using @rules_python//python/cc:current_py_cc_headers and @rules_python//python/cc:current_py_cc_libs) and remove local_config_python entirely. At that point, the name "python_configure" will no longer make sense and should be replaced by, say, "pybind11_configure". Notably, only Bzlmod users need to "configure" something (just getting pybind11 itself) whereas WORKSPACE users no longer need to "configure" anything.

[junyer]

Thanks to @rickeylev's advice, I'm going to simplify things even further: by turning @pybind11 into a hidden implementation detail, Bzlmod users won't need to "configure" anything either! Also, the pybind11_embed target no longer makes sense and will be removed in favour of equivalent support in the next release of rules_python. All of this will be mildly disruptive, but for the best both for pybind11_bazel itself (drastically simpler implementation!) and, more importantly, for pybind11_bazel users.

[junyer]

Reopening since there are further PRs to come.