Don't print :__pkg__ for package rules, since that's the default.

To reduce clutter a little.

Test: m nothing
Change-Id: I0d2d86f00c3d38e603d6f58c4b1dd9a4170bf819

android/visibility.go

@@ -140,7 +140,7 @@ func (r packageRule) matches(m qualifiedModuleName) bool {
 }
 
 func (r packageRule) String() string {
-	return fmt.Sprintf("//%s:__pkg__", r.pkg)
+	return fmt.Sprintf("//%s", r.pkg) // :__pkg__ is the default, so skip it.
 }
 
 // A subpackagesRule is a visibility rule that matches modules in a specific package (i.e.