Add some crates sources to "rust-src" distribution.

[taeguk]

In IntelliJ-Rust and rls-vscode, rust-src component is used to acquire standard crates sources.
But, in currently, librustc, librustc_plugin, and libsyntax are emitted in the list below.

rust/src/bootstrap/dist.rs

Line 889 in ad43389

let std_src_dirs = [

So, syntax highlighting and code completion are not working when using those crates in IntelliJ-Rust or rls-vscode.

There was similar issue #55279 about libproc_macro and it was fixed in #55280.
How about adding those crates (librustc, librustc_plugin, and libsyntax) to the list above, too?

Related issue: intellij-rust/intellij-rust#2623

[ishitatsuyuki]

Related: #42667

[eddyb]

Is this still a concern? I don't think the current set of crates (librustc and librustc_ast) are enough to cover plugin dependencies anymore but I don't think we've heard back from anyone about it.

IMO the solution used here (to add compiler crates to rust-src) is not ideal, and we should probably have any compiler sources available through a different rustup component, if we actually need them.

For now we should probably just remove librustc and librustc_ast from the list.

cc @Mark-Simulacrum

[RalfJung]

Also see #69631