Fix goPath trimming in NewPackage func

[hectorj]

Hi!

I have a list of paths as GOPATH, and I was getting absolute paths for all the goPackagePath values in my generated deps.nix.

So I refactored the gopath trimming used by currentPackage() into a separate function, and used that in NewPackage() instead of the strings.TrimPrefixthat was there.

[kamilchm]

Hi, thx for PR.
Could you provide an example of GOPATH that you want to support?

[hectorj]

Sure. In my case it was a 2 paths list : /home/hectorj/myProjectRepo/go:/home/hectorj/go.

And my issue was that I ended up with values like /home/hectorj/myProjectRepo/go/src/github.com/someUser/someRepo as deps' goPackagePath instead of just github.com/someUser/someRepo

[kamilchm]

Thx, I'll look at it shortly.