Useflag for Rust sources

[leonardohn]

Some tools used on editors and IDEs like racer, rls and rust-analyzer requires the Rust sources (rustup component rust-src) to be available in the system. Until recently, they were implicitly installed by using the rls useflag, however, it seems not to be the case since one of the recent Rust ebuild versions. Considering that, I would like to propose the creation of a new useflag to explicitly install the sources, so that rls would no longer be required for the purpose.

[mmatous]

I would like the scope of this request to be broadened for all reasonable components listed here. I'm currently trying to build the Cranelift-based backend and that requires rust-src, rustc-dev and llvm-tools-preview.