Support for different dependency lists in image.yaml

CI build containers consumed by ASWF projects are built from `ci-PROJECT/Dockerfile` which are itself generated from `ci-PROJECT/image.yaml` using:

```
aswfdocker dockergen
```

The direct or transitive dependencies of an ASWF project can vary over time, so we need to be able to adjust the list of packages that are assembled into a build container based on the VFX Platform year. This may be best achieved by making better use of Conan dependency management.

Start with the ci-PROJECT/image.yaml files and the aswfdocker dockergen entry point that generates each ci-PROJECT/Dockerfile. Trace how dependencies are assembled, then determine how VFX Platform years should select different direct or transitive package lists, potentially through Conan; done means generated build containers use the appropriate dependency set.

Written by the indexing model from the issue text.