CMake: create an alias for KPMcore that fixes its interface

- kpmcore (when used as target "kpmcore") has an interface
  include directory that does not contain the "kpmcore/"
  subdirectory. But the headers it has installed, assume it
  is there (e.g. kpmcore internals use #include <fs/filesystem.h>).
- add an alias at Calamares level that sticks in some more
  includes, adds the relevant WITHOUT_kpmcore when it's
  not there, etc.

CMakeModules/KPMcoreHelper.cmake

@@ -7,28 +7,49 @@
 #
 # Finds KPMcore and consistently sets API flags based on the version.
 #
-if ( NOT KPMcore_searched_for )
+# If KPMcore is not found, still create calamares::kpmcore interface
+# library, which will add definition WITHOUT_KPMcore.
+#
+if(NOT KPMcore_searched_for AND NOT TARGET calapmcore)
     set(KPMcore_searched_for TRUE)
 
-    find_package( KPMcore 21.12.0 )
+    find_package(KPMcore 20.04.0)
     set_package_properties(
-        KPMcore PROPERTIES
+        KPMcore
+        PROPERTIES
         URL "https://invent.kde.org/kde/kpmcore"
         DESCRIPTION "KDE Partitioning library"
         TYPE RECOMMENDED
         PURPOSE "For disk partitioning support"
     )
+
+    # Create an internal Calamares interface to KPMcore
+    # and give it a nice alias name. If kpmcore is not found,
+    # then make a "no KPMcore" library.
+    add_library(calapmcore INTERFACE)
+
+    if(KPMcore_FOUND)
         find_package(Qt5 REQUIRED DBus) # Needed for KPMCore
         find_package(KF5 REQUIRED I18n WidgetsAddons) # Needed for KPMCore
 
-    if( KPMcore_FOUND )
+        target_link_libraries(calapmcore INTERFACE kpmcore Qt5::DBus KF5::I18n KF5::WidgetsAddons)
+        target_include_directories(calapmcore INTERFACE ${KPMCORE_INCLUDE_DIR})
         set(KPMcore_API_DEFINITIONS "")
     else()
         set(KPMcore_API_DEFINITIONS WITHOUT_KPMcore)
     endif()
 
     foreach(d ${KPMcore_API_DEFINITIONS})
-        target_compile_definitions(kpmcore INTERFACE ${d})
+        target_compile_definitions(calapmcore INTERFACE ${d})
     endforeach()
-    target_include_directories(kpmcore INTERFACE ${KPMCORE_INCLUDE_DIR})
+
+    add_library(calamares::kpmcore ALIAS calapmcore)
+else()
+    if(TARGET calapmcore)
+        message(STATUS "KPMcore has already been found")
+        set(KPMcore_FOUND TRUE)
+    else()
+        message(STATUS "KPMcore has been searched-for and not found")
+        set(KPMcore_FOUND FALSE)
+    endif()
 endif()