refactor: move toolchain types to a submodule

[alexeagle]

This unblocks rulesets from providing their own toolchain definitions for things like cargo and clippy, while reusing rules_rust for compilation with rustc.

Later, we can split cargo and clippy to their own toolchains, so we can slim down rustc toolchain and optimize fetches.

Note: this is a second attempt at #3857; the author of that PR lost write access to the PR branch