proposal: relax constraint on requiring module-based main packages

[myitcv]

Now that we have renamed goget to be gobin and introduced an explicit -g flag, I think we can safely relax the constraint that main packages be part of a module.

Reason being, the "get" only happens initially and then subsequently with the explicit -g. So the dependencies of a non-module main package will only "move" when a user does an explicit -g... which is probably not that surprising.