fix: register autodetecting toolchain automatically

@bazel_tools depends on rules_python, but the version is currently
specifies (0.22.0) doesn't register a toolchain by default. This means
the Bazel builtin Python rules aren't usable because no toolchain
is registered for them.

Registering the toolchain directly in @bazel_tools inverts the problem:
a toolchain is now always registered, but will usually always have
precedence over any that rules_python registers (because @bazel_tools
typically comes earlier in the module graph ordering).

Upgrading bazel_tools past to 0.23.0 or higher (which register a Python
toolchain automatically) is possible, but results in many failures in
Bazel CI that are hard to figure out.

To fix, we make rules_python register the autodetecting toolchain.
Later rules_python versions register a different toolchain, but this
at least gives Bazel something it can set as the minimum version that
preserves existing behavior.

Work towards bazelbuild/bazel#20458

Author: rickeylev

MODULE.bazel

@@ -49,3 +49,12 @@ use_repo(
 
 python = use_extension("@rules_python//python/extensions:python.bzl", "python")
 use_repo(python, "pythons_hub")
+
+# Necessary so that some toolchain is available so that the Python rules
+# work in some minimal capacity.
+# The @bazel_tools repo depends on rules_python, so expects rules_python
+# to register a toolchain. @bazel_tools can't register a Python toolchain
+# itself because @bazel_tools will typically come sooner in the module
+# graph BFS order (it is an implicit dep of every module), and would thus
+# override any toolchain rules_python registers.
+register_toolchains("@bazel_tools//tools/python:autodetecting_toolchain")