std/rustc/rustpkg/syntax: Support the `extern mod = ...` form

This commit allows you to write:

 extern mod x = "a/b/c";

which means rustc will search in the RUST_PATH for a package with
ID a/b/c, and bind it to the name `x` if it's found.

Incidentally, move get_relative_to from back::rpath into std::path
rustpkg: Simplify the PkgId struct

Get rid of special cases for names beginning with "rust-" or
containing hyphens, and just store a Path in a package ID. The Rust-identifier
for the crate is none of rustpkg's business.