Handle ( go.mod's ) replace gracefully

[nicocesar]

My go.mod has a specific replace
replace github.com/AdRoll/goamz => github.com/arvados/goamz v0.0.0-20190905141525-1bba09f407ef

and goas fails saying:
2020/08/05 18:13:32 Error: expect package version, got "=>"

When I comment out that line in go.mod, everything works as expected

[nicocesar]

Digging down a little more this seems to come from https://github.com/mikunalpha/go-module that doesn't support replace correctly and the project that was forked from ( https://github.com/uudashr/go-module ) mentions that was deprecated by
https://pkg.go.dev/golang.org/x/mod

The options that I see here:

1. I add better support to https://github.com/mikunalpha/go-module
2. migrate goas to use https://pkg.go.dev/golang.org/x/mod for parsing.

any thoughts @mikunalpha ?

[nicocesar]

Created PR #11 to solve 2)

I'll be re-shaping github.com/nicocesar/goas from now on