auto merge of #15319 : alexcrichton/rust/no-crate-id, r=brson

auto merge of #15319 : alexcrichton/rust/no-crate-id, r=brson

This is an implementation of [RFC 35](https://github.com/rust-lang/rfcs/blob/master/active/0035-remove-crate-id.md).

The summary for this PR is the same as that of the RFC, with one addendum:

* Removes the `#[crate_id]` attribute and knowledge of versions from rustc.
* Added a `#[crate_name]` attribute similar to the old `#[crate_id]` attribute
* Output filenames no longer have versions or hashes
* Symbols no longer have versions (they still have hashes)
* A new flag, `--extern`, is used to override searching for external crates
* A new flag, `-C metadata=foo`, used when hashing symbols
* [added] An old flag, `--crate-name`, was re purposed to specify the crate name from the command line.

I tried to maintain backwards compatibility wherever possible (with warnings being printed). If I missed anywhere, however, please let me know!

[breaking-change]

Closes #14468
Closes #14469
Closes #14470
Closes #14471