Simplify toolchain type hierarchy

It feels to me like we have a lot of different "transition" and a whole bunch of supporting code, but we should use a small number of toolchain name types throughout the codebase and proactively resolve to these early/upfront.

It would be good to summarize the design of these and see if some of it can be flattened.

Originally posted by @djc in https://github.com/rust-lang/rustup/issues/5071#issuecomment-5711393546