expose property doesn't work with fullPaths enabled (fixes #850)

If I set `fullPaths: true` on a bundle (which is required by watchify), and also use `expose` on one of the modules in that bundle, the full path clobbers the exposed name and anything that requires the exposed name throws an exception to the effect of `Error: Cannot find module '/foo/bar/baz'` (i.e. the full path).