The commands moved to shared_internal_commands.cmake
will soon be used directly by FetchContent, which cannot
always include the full ExternalProject.cmake file (e.g. it may
be used in CMake script mode).
Issue: #21703
Teach `ExternalProject_Add` and `FetchContent_Declare` to resolve
relative remote URLs provided via `GIT_REPOSITORY`. Add policy
CMP0150 to maintain compatibility.
Fixes: #24211
Co-Authored-By: Craig Scott <craig.scott@crascit.com>