Use output dir for empty packages to be hermetic (#3098)

With an empty lib reproducibility test

go/tools/builders/compilepkg.go

@@ -178,7 +178,12 @@ func compileArchive(
 	defer cleanup()
 
 	if len(srcs.goSrcs) == 0 {
-		emptyPath := filepath.Join(workDir, "_empty.go")
+		// We need to run the compiler to create a valid archive, even if there's
+		// nothing in it. GoPack will complain if we try to add assembly or cgo
+		// objects.
+		// _empty.go needs to be in a deterministic location (not tmpdir) in order
+		// to ensure deterministic output
+		emptyPath := filepath.Join(filepath.Dir(outPath), "_empty.go")
 		if err := ioutil.WriteFile(emptyPath, []byte("package empty\n"), 0666); err != nil {
 			return err
 		}

tests/integration/reproducibility/reproducibility_test.go

@@ -35,7 +35,13 @@ func TestMain(m *testing.M) {
 	bazel_testing.TestMain(m, bazel_testing.Args{
 		Main: `
 -- BUILD.bazel --
-load("@io_bazel_rules_go//go:def.bzl", "go_binary")
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_library(
+	name = "empty_lib",
+	srcs = [],
+	importpath = "empty_lib",
+)
 
 go_binary(
     name = "hello",