ExternalProject: Ignore macOS .DS_Store files in tarball extraction

Do not consider a top-level `.DS_Store` file when deciding whether a
tarball contains exactly one directory whose contents should be used as
the resulting top-level of the extraction.

Fixes #16218.

Suggested-by: Patrice Kouame

Modules/ExternalProject.cmake

@@ -993,6 +993,7 @@ endif()
 #
 message(STATUS \"extracting... [analysis]\")
 file(GLOB contents \"\${ut_dir}/*\")
+list(REMOVE_ITEM contents \"\${ut_dir}/.DS_Store\")
 list(LENGTH contents n)
 if(NOT n EQUAL 1 OR NOT IS_DIRECTORY \"\${contents}\")
   set(contents \"\${ut_dir}\")