Use the libsyntax PkgId parser in Rustpkg, but keep Rustpkg's version smarts.

This fixes a bug where new syntax crate IDs would cause rustpkg to fail to
build crates.
Change some rustpkg tests to use new crate_id syntax.
auto merge of #11086 : metajack/rust/rustpkg-new-crateid-syntax, r=cmr

...arts.

This fixes a bug where new syntax crate IDs would cause rustpkg to fail to
build crates.