Flag suspect update sites

It would be nice if we automatically flagged update sites that:

a) have direct/transient SNAPSHOT dependencies
b) contain libraries that override core update site classes

This would allow users to see which sites could cause problems with their ImageJ/Fiji installation, and help encourage developers to maintain their software.

No files, tests, or entry points are named. Start by locating the update-site discovery and dependency metadata handling in ImageJ/Fiji, then determine how direct or transient SNAPSHOT dependencies and classes overriding core update-site classes can be identified. Done means both categories are automatically flagged for users.

Written by the indexing model from the issue text.