Add proc_macro crate sources to "rust-src" distribution

[vlad20012]

In IntelliJ-Rust we use "rust-src" rustup component to acquire standard crates sources (which we use for all IDE intelligence). And there is not proc_macro crate in the standard "rust-src" distribution. It results to the fact that our IDE know nothing about proc_macro crate and can't provide any IDE features in code that uses it.
So it would be very nice to add "proc_macro" to "rust-src" rustup component distribution =)

Is it enough to just add it to this list?

rust/src/bootstrap/dist.rs

Line 858 in 554b787

let std_src_dirs = [

I see proc_macro has a several dependencies: "libsyntax", "libsyntax_pos", "librustc_errors", "librustc_data_structures". Is it ok to not add them to the list?