Should we split individual APIs into their own packages

[iorubs]

The nexus folder keeps growing and it might be a good idea to split it into separate packages (Note: using internal packages could also be a good idea).

This would require quire a few changes tho:

1. We will need an extra package with an APICaller interface implemented by the existing Client struct to avoid "import cycles".
2. The existing test files would all have to be put in the same place or the test suits would have to be redesigned/duplicated across all internal sub-packages.
3. The way models are imported would change, we might even have to move them to a separate package if we decide to use internal packages.