Allows the `load` graph to be "load what you use" rather than pulling
in all `.bzl` files in a single `def.bzl` file.

This removes the need for a custom `resolved` rule for every toolchain.

Upgrade path is to add the `toolchain_type` to the local repository
rule in `MODULE.bazel` and change the `resolved` target to an alias.

A `toolchain_resolved` repository rule is added as well for toolchains
that do not use local binaries.

This patch hugely simplifies the creation of toolchains downstream.