Java: Cleanup imports of ExternalFlow

[michaelnebel]

In this PR we cleanup the bi-directional framework imports that were introduced for Models as Data.
The bi-directional imports created a huge SCC of dependencies and removing the importa revealed some missing imports in other parts of the code that were relying on the framework SCC in ExternalFlow.qll.
The code has been modified accordingly and bi-directional imports because of other abstract classes have been introduced where needed.

[atorralba]

Mostly looks good to me, thanks for doing this! I added a couple NITs (that you can completely ignore), and one minor fix.

[atorralba]

Wondering if this is worth a DCA run 🤔 (when it works).

[michaelnebel]

Wondering if this is worth a DCA run 🤔 (when it works).

This is definitely worth a DCA run! We need to see if there are any sudden drops in performance (or improvements).
We could risk worse performance, if some bi-directional imports have been overlooked that could cause re-evaluation.
Will not merge before we are able to run DCA.

[michaelnebel]

DCA looks good.

• The overall performance looks acceptable.
• There appears to be some minor differences in a couple of the query execution times (stages), but an overall reduction in the execution time by all stages. This might be a glitch or maybe a change in the execution order.

Merging now.