This will convert the path to the Rust repo to an absolute path. This is
important for the clippy_lints/Cargo.toml file. Otherwise if a relative
path is passed, rst-analyzer won't find the Rust repo, because it starts
the relative path search from the clippy_lints dir, not the
rust-clippy dir where the ra_setup command was run from.
This takes an absolute path to a rustc repo and adds path-dependencies
that point towards the respective rustc subcrates into the Cargo.tomls of
the clippy and clippy_lints crate.
This allows rustc-analyzer to show proper type annotations etc on rustc-internals inside the clippy repo.
Usage: cargo dev ra-setup /absolute/path/to/rust/
cc https://github.com/rust-analyzer/rust-analyzer/issues/3517
cc https://github.com/rust-lang/rust-clippy/issues/5514
