Refactoring Env

Hiya, continued the refactoring of our Environement constructor.

First of all, _extraction_. I extracted the "Resolver" (logic to find generator on disk), I extracted the storing/requesting logic in its own module so it doesn't obfuscate the business logic of the methods requesting the generator store.

Extracting those simplify bits of code, but mostly, it open field to add an Adapter module to abstract the user interface, and will help refactor the Resolver so it bases it research on NPM `ls` rather than manual search through folders.

Then I deleted `env.remote` and `env.engine`. Remote was broken for a while, my guess it is a relicat from an old feature. And `engine` simply wasn't used (and didn't make too much sense with the current Yeoman context).