Deleted Snyk projects not removed from cache

[ebickle]

The sync() function in cli.py will load all existing repos from the cache (load_watchlist()), list all GitHub repositories, then list all Snyk projects matching those repositories and add or update information as appropriate.

However, it does not check whether a Snyk project no longer exists for a repository and, if so, remove it from the cache. Regardless of the intent, it leads to multiple issues:

• If a Snyk project is accidentally deleted or intentionally deleted to force the importer to reimport it, the importer might believe it's already imported and skip it.
• The "tag" command will fail with a 404 when it tries to add a tag to a project that does not exist.

The later is causing our synchronization to fail - it continually tries to add tags to projects that don't exist and crashes.