UpstreamSource: move version guessing logic to PkgPolicy

PkgPolicy is more logical context, for example if guessing version from
a plain filename and not a real file.

Also, changes guess_version() to always return a tuple to simplify
checking its return value.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Conflicts:
	gbp/pkg/__init__.py
	tests/06_test_upstream_source.py