Fix Collect USD Layers refactor and transfer productGroup to the extra product

[BigRoy]

Changelog Description

This fixes the Collect USD Layers logic where Explicit layers generated within the current solaris graph will turn into their own products.

Additional info

You'll need to have an "Explicit" layer in your solaris graph that will be saved along:

Testing notes:

1. Create a Solaris graph which generates an Explicit Save Layer as part of the graph.

E.g. this Sop Import generating a new 'layer' merged in:

Note the Layer Save Path - it must be explicit otherwise Houdini's USD ROP node will error.

2. Publish USD
3. Should work with and without USD contribution workflow

[MustafaJafar]

This is amazing!