fix #296 - add go modules support

[s-kostyaev]

Add go modules support with golang.org/x/tools/go/packages

[Julusian]

This works nicely for me.

I had to change the generate command to //go:generate go run gopkg.in/src-d/go-kallax.v1/generator/cli/kallax gen. It would be good to document this requirement in the readme.

And for anyone else wanting to test I added a temporary line to my go.mod file to use this PR: replace gopkg.in/src-d/go-kallax.v1 => github.com/s-kostyaev/go-kallax v1.3.6-0.20190705185553-021d805e3744

[iorlas]

Confirming, works good. It just needs to be confirmed to be working for non-mod projects for backward compat

[felipemfp]

@roobre, can you help to fix the failing tests?