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.

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) {