This fixes relative path modules not being correctly vendored by
parsing go.mod in Nix and vendoring them into the correct directory in
the Nix sandbox.
Ideally this should also come with a pluggable source filtering
mechanism and/or maybe use any present gitignore files like
poetry2nix does.
To use this new feature you have to pass a new `pwd` param to
`buildGoApplication` as such:
```
buildGoApplication {
pname = "foo";
version = "bar";
pwd = ./.;
src = lib.cleanSource ./.;
}
```
Fixes#2
