unused: don't track objects in other packages that use us

When converting between two struct types, the fields use each other. However, we mustn't track the fields in a struct type from another package, as they should always be considered used, and we may end up creating nodes for them without ever recording uses. The old implementation of unused behaved correctly; we accidentally dropped a check. Closes gh-1360. (cherry picked from commit 4ceb5b2)

Author: dominikh

unused/unused.go

@@ -476,8 +476,13 @@ func isIrrelevant(obj types.Object) bool {
 }
 
 func (g *graph) use(used, by types.Object) {
-if g.opts.ExportedIsUsed && used.Pkg() != g.pkg || used.Pkg() == nil {
-return
+if g.opts.ExportedIsUsed {
+if used.Pkg() != g.pkg || used.Pkg() == nil {
+return
+}
+if by != nil && by.Pkg() != g.pkg {
+return
+}
 }
 
 if isIrrelevant(used) {