refactor: make bzlmod directly aware of created toolchain repo names

[rickeylev]

This makes python_register_toolchains return the repo names it created, which allows
the bzlmod code to be directly aware of the repos that were created instead of having
to rely on assuming the names via the platform keys.

This is to facilitate python_register_toolchains creating a more arbitrary subset
of platform-specific repos.

Work towards #2081

[rickeylev]

Ready for PTAL; dependency PR merged

[aignas]

LGTM

[aignas]

Suggested change

	for i, toolchain_info in enumerate(py.toolchains):
	is_last = (i + 1) == len(py.toolchains)
	all_toolchains = zip(
	[False] * len(py.toolchains),
	py.toolchains,
	)[:-1]
	all_toolchains.append(
	(True, py.toolchains[-1])
	)
	for set_python_toolchain_constraint, toolchain_info in all_toolchains:

You could do this, I am curious what is more readable.

Also, I can't remember if the last toolchain is still special because we always know the python_version flag setting, so the comment/logic may be outdated. At least when I was bugfixing code in this location I think that the case when the python_version does not match the preceding entries never happens in bzlmod.