Refactor Env#lookup

First stab at simplifying the generator lookup.

I removed some abstraction overhead (prefix, suffix, paths, lookups) to privilege simple values. This removed a LOT of code which wasn't useful.

Also, looking up generator now rely on NPM `ls` functionality and works great with linked packages.

I've also added unit test, so we now at least got some coverage on this very important part of the system. This may have slowed down the test suite quite a bit as it requires installing generators. I now rely on some real generator to be installed, and only one "generator-dummy". It may be better to create a second generator-dummy to test both local and global.

Also, wish to run some extra tests around performances and make sure there haven't been any regression. So, looking for feedback and code review :)

(commit messages are also ugly, I'll rewrite them.)