Rollup merge of #41546 - cuviper:reduced-rust-src, r=alexcrichton

Rollup merge of #41546 - cuviper:reduced-rust-src, r=alexcrichton

Shrink the rust-src component

Before this change, the installable rust-src component had essentially the same contents as the rustc-src dist tarball, just additionally wrapped in a rust-installer.  As discussed on [internals], rust-src is only meant to support uses for the standard library, so it doesn't really need the rest of the compiler sources.

Now rust-src only contains libstd and its path dependencies, which roughly matches the set of crates that have rust-analysis data.  The result is **significantly** smaller, from 36MB to 1.3MB compressed, and from 247MB to 8.5MB uncompressed.

[internals]: https://internals.rust-lang.org/t/minimizing-the-rust-src-component/5117