treewide: Use StringSet alias consistently instead of std::set<std::string>

The intention is to switch to transparent comparators from N3657 for
ordered set containers for strings and using the alias consistently
would simplify things.

src/libstore/dummy-store.cc

@@ -22,7 +22,7 @@ struct DummyStoreConfig : virtual StoreConfig {
           ;
     }
 
-    static std::set<std::string> uriSchemes() {
+    static StringSet uriSchemes() {
         return {"dummy"};
     }
 };