Help: Update cmake-packages.7 examples to import targets only once

Package configuration files should load their targets files at most once
in a given scope.  Update our ClimbingStats example to show how to do
this.

Help/manual/cmake-packages.7.rst

@@ -385,9 +385,12 @@ In this case, the ``ClimbingStatsConfig.cmake`` file could be as simple as:
 
 .. code-block:: cmake
 
-  include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
+  if(NOT TARGET Upstream::ClimbingStats)
+    include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
+  endif()
 
-As this allows downstreams to use the ``IMPORTED`` targets.  If any macros
+This allows downstreams to use the ``IMPORTED`` targets, and guards
+against importing the targets more than once.  If any macros
 should be provided by the ``ClimbingStats`` package, they should
 be in a separate file which is installed to the same location as the
 ``ClimbingStatsConfig.cmake`` file, and included from there.
@@ -454,7 +457,9 @@ dependencies of a package should be found in the ``Config.cmake`` file:
   include(CMakeFindDependencyMacro)
   find_dependency(Stats 2.6.4)
 
-  include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
+  if(NOT TARGET Upstream::ClimbingStats)
+    include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
+  endif()
   include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsMacros.cmake")
 
 The ``find_dependency`` macro also sets ``ClimbingStats_FOUND`` to ``False`` if
@@ -471,7 +476,9 @@ be true. This can be tested with logic in the package configuration file:
   include(CMakeFindDependencyMacro)
   find_dependency(Stats 2.6.4)
 
-  include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
+  if(NOT TARGET Upstream::ClimbingStats)
+    include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsTargets.cmake")
+  endif()
   include("${CMAKE_CURRENT_LIST_DIR}/ClimbingStatsMacros.cmake")
 
   set(_supported_components Plot Table)