CMAKE: Make CMake config idempotent w.r.t. target discovery and creation

cmake/ucx-targets.cmake.in

@@ -7,6 +7,34 @@
 get_filename_component(prefix "${CMAKE_CURRENT_LIST_DIR}/../../.." ABSOLUTE)
 set(exec_prefix "@exec_prefix@")
 
+set(_ucxx_targets_defined "")
+set(_ucxx_targets_not_defined "")
+set(_ucxx_expected_targets "")
+foreach(_ucxx_expected_target IN ITEMS ucx::ucs ucx::uct ucx::ucp)
+  list(APPEND _ucxx_expected_targets "${_ucxx_expected_target}")
+  if(TARGET "${_ucxx_expected_target}")
+    list(APPEND _ucxx_targets_defined "${_ucxx_expected_target}")
+  else()
+    list(APPEND _ucxx_targets_not_defined "${_ucxx_expected_target}")
+  endif()
+endforeach()
+unset(_ucxx_expected_target)
+if(NOT _ucxx_targets_not_defined)
+  unset(_ucxx_targets_defined)
+  unset(_ucxx_targets_not_defined)
+  unset(_ucxx_expected_targets)
+  unset(CMAKE_IMPORT_FILE_VERSION)
+  return()
+endif()
+if(NOT _ucxx_targets_defined STREQUAL "")
+  string(REPLACE ";" ", " _ucxx_targets_defined_text "${_ucxx_targets_defined}")
+  string(REPLACE ";" ", " _ucxx_targets_not_defined_text "${_ucxx_targets_not_defined}")
+  message(FATAL_ERROR "Some (but not all) ucx targets were already defined.\nTargets Defined: ${_ucxx_targets_defined_text}\nTargets not yet defined: ${_ucxx_targets_not_defined_text}\n")
+endif()
+unset(_ucxx_targets_defined)
+unset(_ucxx_targets_not_defined)
+unset(_ucxx_expected_targets)
+
 add_library(ucx::ucs SHARED IMPORTED)
 
 set_target_properties(ucx::ucs PROPERTIES