scanner: allow running without versionCode and as API
scanner: allow running without versionCode and as API

This lets `fdroid scanner my.package.name` run without requiring that the
versionCode is also specified.  It also allows scanner.scan_source() to be
called as a function in the public API of fdroidserver.

f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver