[refactor] Split Database.cpp into db subdirectory

* Database.hpp is still the only external include file (moved to db/)
* The header is still uppercase to match its namespace
* db/internal.hpp is the shared header for the DB source files
* Added -Isrc/ compile flag for src-relative include paths
* Hoisted CHDR above CSRC in Makefile (it was bothering me)
* make clean now removes all objects in the subdirectories as well

src/RacingManager.cpp

@@ -4,7 +4,7 @@
 #include "PlayerManager.hpp"
 #include "MissionManager.hpp"
 #include "ItemManager.hpp"
-#include "Database.hpp"
+#include "db/Database.hpp"
 #include "NPCManager.hpp"
 
 std::map<int32_t, EPInfo> RacingManager::EPData;