MemoryPack generated unions don't work with nested interfaces

[pixsperdavid]

The following gist shows a semi-complex hierarchy of nested interfaces and records:

https://gist.github.com/pixsperdavid/a1eda8c5742e9c712860d5a93a46cc9b

This fails to build as MemoryPack is generating a partial interface IV1 which is not enclosed in a partial nested interface, causing a duplicate definition.

As I added the nested classes support originally this is my oversight, working on a PR currently to address.