Expose internals

[mankyKitty]

From commit messages:

This makes it easier to combine/adapt overrides
in downstream dependencies. Without this, users
would need to override everything required by nix-thunk.
In case of a future update to nix-thunk, users would then
need to go through all of their own dependencies and
resolve them.

With this mechanism exposed, downstream users can get a
working set of pacakges from nix-thunk and need only
explicitly mention the actual compatibility/conflict issues
in their own builds.