Improve GOPATH handling (part 2)

This is the continuation of #658.

TODO:

- [ ] I still need to set the GOPATH at first project in IDEA et al. and save it to this configuration but after that it should be ok. 
- [ ] perform the check that the GOPATH we set contains the "src/" (and maybe "bin/") folder in it since that's about the only indication we can have for it to be correctly set.

Also, since there aren't any complaints about the completion from GOPATH itself, I'll leave it like this for the moment, on other changes to the way we handle it.