Change package name to main from src when creating a file in src folder

This changes the default behavior to create a package called ``` main ``` when creating a file in the ``` src ``` folder rather that name it ``` src ```.
It's something I've wanted to do in a long time and it seems at least one user bumped into it, @asampal in https://github.com/mtoader/google-go-lang-idea-plugin/issues/309#issuecomment-30096605