As part of adding support for threat-models to Python/JS (see
github#17203), we ran into some trouble
with name clashes.

Naming in existing languages supporting threat-models:
- `SourceNode` (for QL only modeling)
- `ThreatModelFlowSource` (for active sources from QL or data-extensions)

However, since we use `LocalSourceNode` in Python, and `SourceNode` in
JS (for local source nodes), it seems a bit confusing to follow the same
naming convention as other languages, and we had to come up with new names.

Initially I used `ThreatModelSource` for the "QL only modeling", but
that meant that we needed a new name to represent the active sources
coming from either QL or data-extensions... for this I came up with
`ActiveThreatModelSource`, and I really liked it. To me, it's much
clearer that this class only contains the currently active threat
model sources.

So to align languages, I got approval from @michaelnebel to rename the
existing classes.