Move get_compression() out of pkg.PkgPolicy class

Move get_compression() out of pkg.PkgPolicy class

Renames the function to parse_archive_filename() and changes it's
return values. Filename parsing is merely generic functionality, not
tied to any packaging policy.

The function now returns the base name of the file (that is, filename
without, archive and compression extensions), archive format and
compression method. Adds supported archive formats 'tar' and 'zip' and
file extension aliases, e.g. 'tgz'.

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