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
In rustpkg, pass around sysroot; in rustpkg tests, set the sysroot
manually so that tests can find libcore and such.
With bonus metadata::filesearch refactoring to avoid copies.
