feat(nix/buildGo): expose gopkg attribute on buildGo.package

This makes the derivations more uniform, since both third-party
packages generated by buildGo.external and native buildGo.package
expose their libraries as gopkg attributes now.

Change-Id: I547f9860082f36e3300139bf67613eb4fc600d24
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1623
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

nix/buildGo/default.nix

@@ -82,13 +82,19 @@ let
     asmPack = ifAsm ''
       ${go}/bin/go tool pack r $out/${path}.a ./asm.o
     '';
-  in (runCommand "golib-${name}" {} ''
+
+    gopkg = (runCommand "golib-${name}" {} ''
       mkdir -p $out/${path}
       ${srcList path (map (s: "${s}") srcs)}
       ${asmBuild}
       ${go}/bin/go tool compile -pack ${asmLink} -o $out/${path}.a -trimpath=$PWD -trimpath=${go} -p ${path} ${includeSources uniqueDeps} ${spaceOut srcs}
       ${asmPack}
-  '') // { goDeps = uniqueDeps; goImportPath = path; };
+    '') // {
+      inherit gopkg;
+      goDeps = uniqueDeps;
+      goImportPath = path;
+    };
+  in gopkg;
 
   # Build a tree of Go libraries out of an external Go source
   # directory that follows the standard Go layout and was not built