scanner: split up code paths for scanning APK/source code
The `options.appid` list in scanner.main is currently used for deciding on whether to abort early after scanning binaries (APKs) or whether scanner should also scan source code. It would make sense to refactor this for better readability/maintainability.
followup to: https://gitlab.com/fdroid/fdroidserver/-/merge_requests/1776#note_3064864174
issue