Rollup merge of #95504 - jyn514:library-alias, r=Mark-Simulacrum

Add `x {check,build,doc} {compiler,library}` aliases.

While working on https://github.com/rust-lang/rust/pull/95503, I realized that it will interfere with existing command lines:
Currently people run `x build library/std` expecting it to "add all library crates to the sysroot",
but after that change, it will *only* build `libstd` and its dependencies (and add them to the sysroot), not libtest or libproc_macro.

That will work for local testing in most cases, but could be confusing. Even if not, though, I think `x build library` is more clear about what actually happens than the current `x build library/std`.

The intended end goal is something like:
- For check/build/doc, we have library + compiler aliases, which correspond to basically "most possible" for that piece. This is the intended path of entry (rather than library/test or similar as today) for when you just want the thing to work -- for example, getting a compiler that is "crates.io-compatible" would be roughly `x.py build library`). #95504
- Specific crate invocations build up to that crate, which means that if you don't care about tests you probably want x.py build library/proc_macro or library/std for faster build times. #95503

Note that this is already implemented today for the `doc` command and seems to work pretty well in practice.

I plan to change the dev-guide and various instructions in the README to `build library` once this is merged.

`@rustbot` label +A-rustbuild

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