CMake: be more verbose on developer machines

This change is relevant only if you build Calamares and some out-of-tree
modules (e.g. calamares-extensions) on the same machine where CMake
caches files in the developer's ~/.cmake/packages . If the user
packages registry kicks in the include files won't be found, and
the targets are not defined.

CalamaresConfig.cmake.in

@@ -23,6 +23,9 @@
 #
 include(${CMAKE_CURRENT_LIST_DIR}/CalamaresConfigVersion.cmake)
 include(${CMAKE_CURRENT_LIST_DIR}/CalamaresTargets.cmake)
+if (NOT TARGET Calamares::calamares OR NOT TARGET Calamares::calamaresui)
+    message(FATAL_ERROR "Calamares found with missing CMake targets")
+endif()
 
 ### Dependencies
 #