Shorten swift-internal:// to swift://

The internal/external distinction can be handled with meta-data
instead of encoding it within the url. So we can simply start using
swift://.

This patch keeps the swift-internal:// option around until we decide
it can simply be removed.

Change-Id: I5ea3d93319daf426539abc78ae826b6594035df9
Implements: blueprint swift-url-proto-cleanup