refactor: Move module type string into modules

This allows us to identify module by their type and it is also better to
store the module type as part of the module instead of having it
hardcoded in factory.hpp

src/modules/date.cpp

@@ -1,6 +1,6 @@
 #include "modules/date.hpp"
-#include "drawtypes/label.hpp"
 
+#include "drawtypes/label.hpp"
 #include "modules/meta/base.inl"
 
 POLYBAR_NS
@@ -93,6 +93,6 @@ namespace modules {
     wakeup();
     return true;
   }
-}
+}  // namespace modules
 
 POLYBAR_NS_END