This is a major reworking of git-env.eclass to make it better at cross-compiling and generally easier to use.
I considered adding a go-env_run_as_target_or_build() helper for the docker-cli case, where you can either use `go run -exec` or rebuild everything for the build system, but the savings made by the former weren't worth the extra complexity, and it can take longer in some cases. The plan is to move the go-env_set_compile_environment() call from go-module's src_unpack to src_configure, but that will be done in a subsequent EAPI 9 bump. We can't do it yet because of dependent eclasses not being ready. The EAPI 9 bump will probably also set CGO_ENABLED=1 globally, but I need to do more investigation around that.
