CMake: target kpmcore doesn't exist elsewhere, so check was failing

Consumers of calapmcode couldn't tell if KPMcore was found.
Pass it around as a property of calapmcore, rather than
anything else.

FIXES #2026

CMakeModules/KPMcoreHelper.cmake

@@ -34,13 +34,16 @@ if(NOT TARGET calapmcore)
         target_include_directories(calapmcore INTERFACE ${KPMCORE_INCLUDE_DIR})
         # If there were KPMcore API variations, figure them out here
         # target_compile_definitions(calapmcore INTERFACE WITH_KPMcore)
+        set_target_properties(calapmcore PROPERTIES KPMcore_FOUND TRUE)
     else()
         target_compile_definitions(calapmcore INTERFACE WITHOUT_KPMcore)
+        set_target_properties(calapmcore PROPERTIES KPMcore_FOUND FALSE)
     endif()
 
     add_library(calamares::kpmcore ALIAS calapmcore)
 else()
-    if(TARGET kpmcore)
+    get_target_property(KPMcore_FOUND calapmcore KPMcore_FOUND)
+    if(KPMcore_FOUND)
         message(STATUS "KPMcore has already been found")
         set(KPMcore_FOUND TRUE)
     else()