rapids_cpm_find is more invariant as one would expect

Fixes rapidsai#49

rapids_cpm_find now looks at the `GLOBAL_TARGETS` entries and
wil not call CPMFindPackage if any of them already exist.

This makes it safe to call `rapids_cpm_find` no matter what
existing targets already exist. therefore users of rapids-cmake
don't need to do manual `if(TARGET ...)` checks, resulting
in missing packages in the export set.

rapids-cmake/cpm/find.cmake

@@ -89,14 +89,25 @@ function(rapids_cpm_find name version)
     message(FATAL_ERROR "rapids_cpm_find requires you to specify CPM_ARGS before any CPM arguments")
   endif()
 
-  CPMFindPackage(NAME ${name} VERSION ${version} ${RAPIDS_UNPARSED_ARGUMENTS})
+  set(package_needs_to_be_added TRUE)
   if(RAPIDS_GLOBAL_TARGETS)
-    include("${rapids-cmake-dir}/cmake/make_global.cmake")
-    rapids_cmake_make_global(RAPIDS_GLOBAL_TARGETS)
+    foreach(target IN LISTS RAPIDS_GLOBAL_TARGETS)
+      if(TARGET ${target})
+        set(package_needs_to_be_added FALSE)
+        break()
+      endif()
+    endforeach()
+  endif()
+
+  if(package_needs_to_be_added)
+    CPMFindPackage(NAME ${name} VERSION ${version} ${RAPIDS_UNPARSED_ARGUMENTS})
   endif()
 
   set(extra_info)
   if(RAPIDS_GLOBAL_TARGETS)
+    include("${rapids-cmake-dir}/cmake/make_global.cmake")
+    rapids_cmake_make_global(RAPIDS_GLOBAL_TARGETS)
+
     set(extra_info "GLOBAL_TARGETS")
     list(APPEND extra_info ${RAPIDS_GLOBAL_TARGETS})
   endif()