rustc: Support 'companion mod's for crates and directory mods

Under this scheme when parsing foo.rc the parser will also look for
foo.rs to fill in the crate-level module, and when evaluating a
directory module directive it will look for a .rs file with the
same name as the directory.

src/compiletest/compiletest.rc

@@ -1,6 +1,5 @@
 use std;
 
-mod compiletest;
 mod procsrv;
 mod util;
 mod header;