swc-installation-test-2.py: List all causes for or-dependency errors

Also make the and-dependency and or-dependency messages more friendly.
swc-installation-test-2.py: Add long names for virtual-* packages

Give a more user-friendly description of what we're checking for.
swc-installation-test-2.py: Add TornadoPythonPackage for version extraction

Tornado follows Python's example with tornado.version and
tornado.version_info instead of using tornado.__version__.  While we
could parse tornado.version with our usual machinery, take advantage
of the pre-parsed version_info by defining a new class.