Unify `download-*` settings in config.toml

[jyn514]

Right now, there are two different download settings: download-ci-llvm and download-rustc. They are pretty similar and I imagine the knobs we'll want to tweak will be the same for both (e.g. it seems reasonable to add "if-unchanged" to download-ci-llvm now that download-rustc has it: #83368). Additionally, this could be extended without too much trouble to other tools and components, like rustdoc, clippy, etc. It would be nice to combine these together in to one option (maybe download-component). I'd expect the old options to work but give a warning to use the new syntax.

Another feature could be to support multiple requirements, or individual components:

• download-component = "clippy,llvm"
• download-component = "rustc=if-unchanged,llvm=if-available"
• download-component = "if-unchanged,if-available"

cc @Mark-Simulacrum, #81930

[Mark-Simulacrum]

I think unifying the backend implementation is OK. I don't care much about avoiding multiple options in the frontend; I think it's not necessarily bad but also seems like there's not much purpose to it - I guess I'm wondering why multiple options would be all that bad.

[camelid]

Semi-related: There's some inconsistency in naming because there's download-ci-llvm but also download-rustc. Not a big deal, but it seems like -ci- should be added to the latter or removed from the former.

[jyn514]

I don't think this is a very useful change.