Install rust-src, rls, and clippy as needed

[parasyte]

rust-src is required by cargo-xbuild, rls, and clippy.

• rls is going to be used by the "rls" command: Add "rls" command #4
• clippy is going to be used by rls

[gogo2464]

@parasyte where do I install? On the Cargo.toml?

[parasyte]

These are development dependencies that cargo-n64 needs (or will need) to run certain commands. The installation instructions are included in the readme: https://github.com/rust-console/cargo-n64#installation

The purpose of this ticket is to install these dependencies automatically when requested, or tell the user what needs to be installed when a dependency is not available.