Some major changes have happened to external flows.
In the past, when externalizing flow snippets, the actors had to be wrapped in an InstantiatableStandalone/Source/Transformer/Sink. This requirement was historical (only introduced an additional layer of complexity) and has now been removed, as well as the InstantiatableStandalone/Source/Transformer/Sink actors. These no-longer available actors will get replaced automatically with sensible actors (Standalones/SequenceSource/SubProcess/Sequence) in order no to break any existing external flows.
I introduced a range of new actors to the adams-meta module for loading external actors: IncludeExternalStandalone/Source/Transformer/Sink. In contrast to the External... actors in the adams-core module, these replace themselves with actual external flow snippet the first time the flow gets setup. This is the more efficient way of using external flows if there is no need for variables attached to the flow file option.