nongnu: Move nonguix-container to its own module.

This MR's purpose is to migrate nonguix-container and related code to a separate module, introducing as little changes as possible to the internal logic.

This is a shot at making nonguix-container's machinery package agnostic. Since it's an open theme in the community (see #79 #93 #154 or !246 (closed) ) I leave the burden of this refactoring to future nonguixers :) .