What if I just want to #include LiquidCrystal.h without knowing where it lives?
You can't.
so I have to copy LiquidCrystal.h and LiquidCrystal.cpp to my custom library folder and use:
No. This implies that you do know where LiquidCrystal.h lives.
You library and LiquidCrystal should have the same parent. So, you should be able to
#include <..\LiquidCrystal\LiquidCrystal.h>
in your library.
In your sketch, you need
#include <YourLibrary.h>
#include <LiquidCrystal.h>