Hosting dependency tarballs for Go modules

A recent version of go-module.eclass now requires packages to provide dependencies as a dependency tarball instead of importing the contents of go.sum into EGO_SUM. Citing go-module.eclass(5):

Here is an example of how to create a dependency tarball. The base directory in the GOMODCACHE setting must be go-mod in order to match the settings in this eclass.

$ cd /path/to/project
$ GOMODCACHE="${PWD}"/go-mod go mod download -modcacherw
$ tar -acf project-1.0-deps.tar.xz go-mod

and:

EGO_SUM

This is replaced by a dependency tarball, see above for how to create one.

When emerging a package which uses EGO_SUM, a QA warning is emitted, saying that this will become a fatal error in the future. Thus, we need some way to host these dependency tarballs.