project(Foo NONE)

cmake_minimum_required(VERSION 3.14)

set(MYMODDIR "${CMAKE_CURRENT_SOURCE_DIR}/Modules")
set(MYMODDIR_ALT "${CMAKE_CURRENT_SOURCE_DIR}//Modules")

file(WRITE ${MYMODDIR}/FindFoo.cmake "
    message(FATAL_ERROR \"Module found in ${MYMODDIR_ALT} !\")
")

set(CMAKE_MODULE_PATH ${MYMODDIR_ALT})

find_package(Foo)