test(bzlmod): add python.toolchain unit tests

[aignas]

With this PR we get rudimentary unit tests for the python bzlmod extension
which allows us to unit test the override behaviour that will become more
complex soon.

Summary:

• refactor: inline the python_register_toolchains
• refactor: use toolchain_info to call python_register_toolchains
• refactor: move the registration out of the main loop
• refactor: split the parsing of the modules to a separate function
• test(bzlmod): add python.toolchain module parsing tests

Work towards #2081