Follow-up from "cmake: switch to project repository"

The following discussion from !2 (merged) should be addressed:

• @ben.boeckel started a discussion:

  A separate MR, but can we have another CMake file that each includes (say "${CMAKE_CURRENT_LIST_DIR}/catalyst-contract.cmake) that provides a function that factors out the majority of the common stuff between each script? Or do we download exactly this script and use it as-is?