go-packages: further simplification

This change is the result of replacing:

    buildGoPackage rec \{\s+rev\s+=\s+"(.*)";\s+name\s+=\s+"(.*)-\$\{stdenv.lib.strings.substring 0 7 rev\}";\s+goPackagePath\s+=\s+"github.com/(.*)/\2";\s+src\s+=\s+fetchFromGitHub\s+\{\s+inherit rev;\s+owner\s+=\s+"\3";\s+repo\s+=\s+"\2";\s+sha256\s+=\s"(.*)";\s+\};\s+\};

with:

    buildFromGitHub {\n    rev    = "$1";\n    owner  = "$3";\n    repo   = "$2";\n    sha256 = "$4";\n  };

in pkgs/top-level/go-packages.nix.

I also inherited `rev` in `buildFromGitHub` to prevent a mass rebuild of Go packages, even though it's unused.