Tweak how hyphens in crate names are allowed

[alexcrichton]

• remove the quote syntax from extern crate, require an identifier to be used instead - rustc: Add support for extern crate foo as bar #23546
• Prevent duplicate uploads of foo-bar and foo_bar to crates.io change
• Automatically translate - to _ in Cargo for passing the --extern flag - Normalize hyphens to underscores in crate names cargo#1443

As an interim step, the compiler should continue to accept extern crate "foo-bar" as ... both in terms of quotes and in terms of matching the identifier foo_bar as well. This should help reduce the breakage a little.

triage: P-backcompat-lang (1.0 beta)

[alexcrichton]

Er, tracking issue for rust-lang/rfcs#940