Auto merge of #54116 - eddyb:extern-prelude, r=petrochenkov

rustc_resolve: allow only core, std, meta and --extern in Rust 2018 paths.

As per https://github.com/rust-lang/rust/issues/53166#issuecomment-419265401:
* Rust 2018 imports can no longer refer to crates not in "extern prelude"
  * `::foo` won't load a crate named `foo` unless `foo` is in the "extern prelude"
  * `extern crate foo;`, however, remains unchanged (can load arbitrary crates)
* `--extern crate_name` is added (note the lack of `=path`) as an unstable option
  * adds `crate_name` to the "extern prelude" (see above)
  * crate is searched in sysroot & library paths, just like `extern crate crate_name`.
  * `Cargo` support will be added later
* `core`, `std` and ~~`proc_macro`~~ `meta` are *always* available in the extern prelude
  * warning for interaction with `no_std` / `no_core` will be added later
  * **EDIT**: `proc_macro` was replaced by `meta`, see https://github.com/rust-lang/rust/issues/53166#issuecomment-421137230
    * note that there is no crate named `meta` being added, so `use meta::...;` will fail, we're only whitelisting it so we can start producing `uniform_paths` compatibility errors

Fixes #54006 (as the example now requires `--extern alloc`, which is unstable).
Fixes #54253 (hit during fixing RLS).

r? @petrochenkov cc @aturon @alexcrichton @Centril @joshtriplett