Some improvements for remote dependency lookups

Makes it possible to lookup a remote dependency's relative without extension.

`require('repo/filename')`

See: https://github.com/tetsuo/builder2.js/blob/patch-js-relative-extension/test/fixtures/js-relative-extension-deps/index.js

Factored out lookup stuff to `builders/lookup.js` not sure if that's the appropriate place for it, couldn't think of somewhere else.