Rollup merge of #148099 - Zalathar:debuggers, r=jieyouxu

Prepare to move debugger discovery from compiletest to bootstrap

For a while I've been wanting to move debugger discovery out of compiletest and into bootstrap, so that bootstrap would be responsible for deciding which debugger to use, and compiletest would faithfully use that debugger with no extra magic (and no horrible config-duplicating hacks).

Making that change is complicated, and eventually I had so many intertwined preparatory changes that I split them off into this PR, so that it can be reviewed and tested as a smaller chunk.

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To avoid scope creep, the changes in this PR try to move code as-is as much as possible, even if the moved code could arguably benefit from further cleanups. And in many cases, that code will need to be further overhauled anyway when discovery steps are actually moved out of compiletest.

Author: GuillaumeGomez