python3: Add canned recipe to invoke filespec processing
This extracts filespec export and processing into
Py3Package/ProcessFilespec.
This also allows the filespec variable to be explicitly set to an empty
value, to bypass filespec processing. (The default filespec is also
available as Py3Package/filespec/Default to be explicitly assigned to
the filespec variable.)
Signed-off-by: Jeffery To <jeffery.to@gmail.com>