Remove crate_id support from `extern crate`

[brson]

It looks like crate ID's are unlikely to carry over into the cargo world. Right now our extern crate syntax supports a form like

extern crate foo = "foo:1.0#path"; // or something similar

Remove the = "string" syntax.

Because we do still need to support renaming, add something like the following:

extern crate foo = bar;

Nominating.

[klutzy]

Some crates (e.g. rust-crypto) currently use hyphen in crate id: extern crate crypto = "rust-crypto";. This change does not permit such ids. (It may be a non-issue if we also accept #14470.)

[brson]

1.0 P-backcompat-lang

[brson]

@klutzy I might question why we allow crates to have names that are not valid idents.

[emberian]

Having rust- in a crate name seems extremely redundant and pointless.

[emberian]

(Similarly, -rs)

[tbu-]

I like the idea of having the possibility to make crates (that is, filenames) which are not strict rust identifiers still usable in rust.