ENH: 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.

Change-Id: Ie21b90aa8a8ffac76c8efa1565453bb29c331527
Merge topic 'ExternalData-no-re-stage'

a894c07 ENH: ExternalData: Do not re-stage staged object files