feature: avoid a fallback to the default-registered toolchains when using config_settings

[novas0x2a]

🚀 feature request

Relevant Rules

This applies to the toolchain registration extensions.

Description

We have a product with multiple parallel python ecosystems, and I'm trying to make them more separate mentally, for our sanity. We're using the idiom described in #3671 (comment) to declare a custom family of toolchains, like:

string_flag(
    name = "python_toolchain_family", 
    build_setting_default = "prebuilt", 
    values = ["prebuilt", "custom"]) 

and then we attached the custom config_setting to our custom toolchains (via target_settings) like:

    native.toolchain(
        ...
        target_settings = [
            "@rules_python//python/config_settings:is_python_3.9",
            "//thirdparty:python_toolchain_family_custom",
        ],
        toolchain = "//thirdparty/Python39:runtime_pair",
        toolchain_type = "@rules_python//python:toolchain_type",
    )

So far so good. However, the problem is, when we request an illegal combination, like:

py_binary(
    python_version = "3.11", 
    config_settings = {
        "//:python_toolchain_family": "custom"
    }
)

(when we didn't register 3.11 as a custom), we then get the default-registered python 3.11 toolchain, instead of getting a toolchain election error.

Describe the solution you'd like

I'd really like a way to unconditionally attach the config to all of the other registered toolchains, too, like

python.override(toolchain_target_settings = ["//:python_toolchain_family_prebuilt"])

So there's a clean separation between our various custom ones and the prebuilt ones. I know this is possible manually with python.single_version_platform_override, but in order for this to work I have to attach it to the entire transitive graph of toolchains.

Describe alternatives you've considered

I'm not super familiar with the rules_python internals, but it might also make sense to attach it to something like config.add_transition_setting. I'm sure there are also probably ways to do this that I haven't thought of :)