Consolidate subcommand parsing in bootstrap

[jyn514]

We should consolidate the subcommand parsing to be consistent - currently, there's many different places where commands are parsed, leading to inconsistencies where they don't match up (so to speak :P).

Originally posted by @jyn514 in #95875 (review)
cc @aswild

@rustbot label +A-bootstrap

[aswild]

[continued from #95875]

The builder::Kind enum seems like it could work, but it's missing variants for Format, Clean, and Setup.

I think just adding the new variants will be simpler than trying to introduce a new type. Are you interested in working on that? I'm happy to mentor :)

Yeah, I'd love to work on this. Last night I got a proof of concept "add a new type" implementation working just to see what it'd be like, but I can change gears and look into adding the needed new variants to builder::Kind. I was a bit intimidated that there might be specific reasons not to have a Builder for the commands that are missing from it (e.g. fmt, clean, and setup).

Another thing I noticed that can be improved in this area, there's several commands that say run './x.py foo -h -v' to see a list of available paths in their help text, but then if you run -h -v there's no paths that get listed. Seems straightforward to generate that message automatically rather than having to keep two lists in sync manually.

@rustbot claim

[jyn514]

@rustbot label +A-rustbuild +E-easy