Intermediate modules that register the same python version as rules_python's default prevent identifying default toolchain

[rickeylev]

🐞 bug report

Affected Rule

python.toolchain

Is this a regression?

Exists as far back as 0.25.0, not sure before that.

Description

If an intermediate module registers the same python version as rules_python's default, then it prevents a default toolchain from being identified.

🔬 Minimal Reproduction

The basic repro is to have an earlier module register the same version as what rules_python sets as the default.

# root MODULE
bazel_dep(name="rules_python")

python = use_extension("@rules_python//python/extensions:python.bzl", "python")
# Two must be registered; one is always treated as is_default=True
python.toolchain(python_version="3.11", is_default=False)
python.toolchain(python_version="3.10", is_default=False)

# rules_python MODULE
python = use_extension("@rules_python//python/extensions:python.bzl", "python")
python.toolchain(python_version="3.11", is_default=True)

The error/output is:

... python/extensions/python.bzl:44:10: WARNING: Ignoring toolchain 'python_3_11' from module 'rules_python': Toolchain 'python_3_11' from module 'middle' already registered Python version 3.11 and has precedence
ERROR: Traceback (most recent call last):
	File ".../python/extensions/python.bzl", line 140, column 13, in _python_impl
		fail("No default Python toolchain configured. Is rules_python missing `is_default=True`?")

Anything else relevant?

Blocking rules_proto_grpc getting onto bcr, but also would affect other intermediate modules

[rickeylev]

It's late so I gotta pack it in. I think to fix this, the logic needs to do two things:

1. Track what version (instead of toolchain struct) was marked as default. Then later look up the toolchain struct for that python version
2. Before skipping a lower-precedence toolchain entirely, check its default-ness so (1) can work.