rustpkg: Eliminate a copy
Makefiles: make rustpkgtest depend on rustpkg executable
compiletest: Remove "remove me after snapshot" code
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
auto merge of #8176 : catamorphism/rust/rustpkg-extern-mod, r=catamorphism

r? @graydon Also, notably, make rustpkgtest depend on the rustpkg executable (otherwise, tests that shell out to rustpgk might run when rustpkg doesn't exist).