buildGoPackage's fetcher don't remove submodules

[yihuang]

ref: https://github.com/golang/go/blob/master/src/cmd/vendor/golang.org/x/mod/zip/zip.go#L212
go get will remove submodules(subdirectories with go.mod in it) after fetch repo, but the fetcher in buildGoPackage simply use fetchgit clone the whole repo.
This has some potential bad effect, also might be an issue when we want to support different versions of submodules from same repo in tools like vgo2nix.
If we simply add postFetch hook to fetchgit to remove directories in repo, the sha256 would not match, an option would be create a wrapper derivation to copy/link the sources (what's the idiomatic way to do this in nix?).

$ GOPATH=`pwd` go get -d github.com/yihuang/test-golang-module-major-version/v2@master
$ ls -1 ./pkg/mod/github.com/yihuang/test-golang-module-major-version/v2@v2.0.2-0.20200924181748-4a13e4bfd5a9
a.go
go.mod
$ git clone https://github.com/yihuang/test-golang-module-major-version.git
$ ls -1 test-golang-module-major-version
a.go
go.mod
sub

[stale]

I marked this as stale due to inactivity. → More info

[yihuang]

since vgo2nix is superseded by gomod2nix, closing.