`x.py check` doesn't allow checking tools other than clippy and rustdoc

[jyn514]

I tried this code:

x.py check src/tools/rustfmt

I expected to see this happen: x.py checks rustfmt

Instead, this happened:

thread 'main' panicked at 'error: no rules matched src/tools/rustfmt', src/bootstrap/builder.rs:225:17

Note I am not asking for the default x.py check to be changed at all, just for it to be possible to check rustfmt in-tree. Right now you have to use x.py build --stage 1 src/tools/rustfmt which compiles the compiler 3 times (two of them unnecessarily - rustfmt uses rustc_ap_*, not the compiler in-tree).

Meta

HEAD is currently 18d855b.

[jyn514]

Fixed in #82208 and #85051.