rpm: enhance logic in guessing the prefix of source archives

rpm: enhance logic in guessing the prefix of source archives

Now a prefix is guessed for all source archives (not for the one and
only "orig archive"). By default it is
'<archive_name>-<archive_version>/' if name and version can be
determined from the filename. If not, the archive basename (i.e.
filename less archive and compression extensions) is used.

In addition, more logic is applied to the "orig archive": gbp examines
the setup macro (if found) for the source and takes the prefix from
there. If some macros (in the '-n' option of the setup macro) cannot be
expanded archive basename us used as the prefix.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>