Core plugin path fix

uses the process.cwd to get the relative path to the core plugins so resolve-dep can find the correct directory from inside other projects. fixes #532

@jonschlinkert (or anyone else on Windows) will you pull this down and test it in a project using assemble to make sure the paths are correct for Windows?
You showed in one of the screenshots that the issue is with only using the `__dirname` without the `path.relative(process.cwd()...`. Then you posted the screenshot with it working correctly.

The "ugly" directory is coming from inside resolve-dep.

BTW... `union` works fine because it flattens the arrays internally. Also, you're screenshots and messages hadn't shown up on the screen before I posted my previous message, so there was some weird timing with what I was referring to there.