Improve error message with bad package ID

[catamorphism]

If you write:

extern mod foo = "https://github.com/catamorphism/test-pkg";

the error message isn't currently useful. It should suggest removing the https:// prefix. (Alternately, this could just be accepted.)

[emberian]

extern crate foo = "https://github.com/catamorphism/test-pkg";

fn main() { }

is an updated testcase. It says:

foo.rs:1:1: 1:63 error: can't find crate for `foo`
foo.rs:1 extern crate foo = "https://github.com/catamorphism/test-pkg";
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

[thestinger]

I don't think this is relevant with the removal of rustpkg. It's not valid without the scheme prefix either.