cmd/go/internal/list: make a copy of the package before zeroing fields

go list -json=<fields> zeroes out the fields in the package struct
that aren't specified. The problem with this is that some of the fields
have references into other fields: specifically, the NoGoError's
Error() function accesses the package struct's Dir field, so if we
clear it out the error will just print out "no Go files in" without a
directory. Instead, make a copy of the package struct before we zero
out the fields so the original values are still there.

For #64946

Change-Id: I95103e91fa0782bb23a86a965d5eb87cb12654c6
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/553795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/list/list.go

@@ -449,13 +449,15 @@ func runList(ctx context.Context, cmd *base.Command, args []string) {
 	if listJson {
 		do = func(x any) {
 			if !listJsonFields.needAll() {
-				v := reflect.ValueOf(x).Elem() // do is always called with a non-nil pointer.
-				// Clear all non-requested fields.
+				//  Set x to a copy of itself with all non-requested fields cleared.
+				v := reflect.New(reflect.TypeOf(x).Elem()).Elem() // do is always called with a non-nil pointer.
+				v.Set(reflect.ValueOf(x).Elem())
 				for i := 0; i < v.NumField(); i++ {
 					if !listJsonFields.needAny(v.Type().Field(i).Name) {
 						v.Field(i).SetZero()
 					}
 				}
+				x = v.Interface()
 			}
 			b, err := json.MarshalIndent(x, "", "\t")
 			if err != nil {

src/cmd/go/testdata/script/list_json_issue64946.txt

@@ -0,0 +1,10 @@
+cd mod
+go list -e -json=ImportPath,Error ./foo
+stdout '"Err": "no Go files in .*(/|\\\\)src(/|\\\\)mod(/|\\\\)foo"'
+
+-- mod/go.mod --
+module example.com/foo
+
+go 1.21
+-- mod/foo/README.md --
+empty