make it installable via go install

[notPlancha]

Hello, I wanted to use this cli tool using go install github.com/Navid2zp/dups, but was unsuccessful. I'm new to go, but I made a fork so this would be possible, and I thought "might as well open a pr". Essentially I just move some stuff around and made the root the main package.

Right now go install github.com/notPlancha/dups works well, but I haven't tested this in any go script. Feel free to reject this, I just wanted to share.