Auto merge of #54658 - petrochenkov:experelude, r=nikomatsakis

Add `extern crate` items to extern prelude

With this patch each `extern crate orig_name as name` item adds name `name` into the extern prelude, as if it was passed with `--extern`.

What changes this causes in practice?
Almost none! After all, `--extern` passed from Cargo was supposed to replace `extern crate` items in source, so if some code has `extern crate` item (or had it on 2015 edition), then it most likely uses `--extern` as well...

... with exception of a few important cases.

- Crates using `proc_macro`. `proc_macro` is not passed with `--extern` right now and is therefore not in extern prelude.
Together with 2018 edition import behavior this causes problems like https://github.com/rust-lang/rust/issues/54418, e.g.
    ```rust
    extern crate proc_macro;
    use proc_macro::TokenStream;
    ```
    doesn't work.
It starts working after this patch.

- `#[no_std]` crates using `std` conditionally, like @aturon described in https://github.com/rust-lang/rust/issues/53166#issuecomment-425219286, and still wanting to write `std` instead of `crate::std`. This PR covers that case as well.
This allows us to revert placing `std` into the extern prelude unconditionally, which was, I think, a [bad idea](https://github.com/rust-lang/rust/issues/53166#issuecomment-425117829).

- Later `extern crate` syntax can be extended to support adding an alias to some local path to extern prelude, as it may be required for resolving https://github.com/rust-lang/rust/issues/54647.

Notes:
- Only `extern crate` items from the root module added to the prelude, mostly because this behavior for items from inner modules would look very strange, rather than for technical reasons.
This means you can opt out from the prelude additions with something like
    ```rust
    mod inner {
        pub(crate) extern crate foo;
    }
    use inner::foo;
    ```
- I haven't updated logic for 2018 import canaries to work fully correctly with this. The cases where it matters are pretty exotic (the `extern crate` item must be "sufficiently macro expanded") and I'd rather spend the time on eliminating the canaries entirely.