This calculates the output hashes in pure Go and removes Nix from
being a run time dependency of gomod2nix.

Additionally it should be faster, but that's untested.

Now that we're not spawning subprocesses like crazy it's far less
relevant to limit concurrency.

And make it run in parallell

Constructs such as
```
replace google.golang.org/grpc => google.golang.org/grpc v1.33.2
replace github.com/gogo/protobuf => github.com/regen-network/protobuf v1.3.3-alpha.regen.1
```

Are perfectly valid and we previously discarded every one but the last of them.

Tests take 1+ hour right now for very little purpose.

We will use this to generate the Github Actions matrix