ExternalData: Do not re-stage staged object files

The ExternalData_LINK_CONTENT option tells ExternalData to convert real
data files it finds into content links and to "stage" the original
content in a ".ExternalData_<algo>_<hash>" file.  However, after a data
object has been staged it is possible that a user-provided pattern in
the "REGEX:" option will later match the staged object file.  We must
not process staged object files even when a user pattern matches them.

Fix the implementation to not match a staged object file as a normal
data file for conversion.  Extend the RunCMake.ExternalData test to
cover this case.

Modules/ExternalData.cmake

@@ -551,7 +551,10 @@ function(_ExternalData_arg_find_files pattern regex)
       set(relname "${entry}")
       set(alg "")
     endif()
-    if("x${relname}" MATCHES "^x${regex}$" AND NOT IS_DIRECTORY "${top_src}/${entry}")
+    if("x${relname}" MATCHES "^x${regex}$" # matches
+        AND NOT IS_DIRECTORY "${top_src}/${entry}" # not a directory
+        AND NOT "x${relname}" MATCHES "(^x|/)\\.ExternalData_" # not staged obj
+        )
       set(name "${top_src}/${relname}")
       set(file "${top_bin}/${relname}")
       if(alg)