cmake: Properly use library detection modules

Instead of importing the `Find*.cmake` modules directly, place them in
the root `cmake/` folder and define it as a CMake Module path. This lets
us use the `find_package` builtin to require any given packages.

CMakeLists.txt

@@ -11,6 +11,8 @@ include(cmake/GetGitRevisionDescription.cmake)
 include(cmake/version.cmake)
 include(GNUInstallDirs)
 
+set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
+
 if(NOT PYTHON_ONLY)
 find_package(BISON)
 find_package(FLEX)

src/lua/CMakeLists.txt

-include(cmake/FindLuaJIT.cmake)
+find_package(LuaJIT)
 
 if (LUAJIT_LIBRARIES)
 	find_program(LUAJIT luajit)