Implement RUST_PATH variation for porting Servo

r? @brson

@metajack requested the ability to violate the "only workspaces can be in the RUST_PATH" rule for the purpose of bootstrapping Servo without having to restructure all the directories. This patch gives rustpkg the ability to find sources if a directory in the RUST_PATH directly contains one of rustpkg's "special" files (lib.rs, main.rs, bench.rs, or test.rs), even if it's not a workspace. In this case, it puts the build artifacts in the first workspace in the RUST_PATH.

I'm not sure whether or not it's a good idea to keep this feature in rustpkg permanently. Thus, I added a flag, ```--use-rust-path-hack```, and only enabled it when the flag is set.