Auto merge of #143548 - Diggsey:db-limit-extern-crate-usage, r=oli-obk

Restrict sysroot crate imports to those defined in this repo.

It's common to import dependencies from the sysroot via `extern crate` rather than use an explicit cargo dependency, when it's necessary to use the same dependency version as used by rustc itself. However, this is dangerous for crates.io crates, since rustc may not pull in the dependency on some targets, or may pull in multiple versions. In both cases, the `extern crate` fails to resolve.

To address this, re-export all such dependencies from the appropriate `rustc_*` crates, and use this alias from crates which would otherwise need to use `extern crate`.

See rust-lang/rust#143492 for an example of the kind of issue that can occur.

Author: bors