rustbuild: Assert extended builds don't dist too much

This extends a test in the previous commit to assert that we don't build
extra rustc compilers even when the "extended" option is set to true.
This involved some internal refactoring to have more judicious usage of
`compiler_for`, added in the previous commit, as well.

Various `dist::*` targets were refactored to be parameterized with a
`Compiler` instead of a `stage`/`host`, and then the various parameters
within the `Extended` target were tweaked to ensure that we don't ever
accidentally ask for a stage2 build compiler when we're distributing
something.

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