feat(nuxt): Support nuxt layers in a better way

[marr]

When using the pinia nuxt module with nuxt layers, there were some things that don't appear to work correctly.

I am trying to solve for a nuxt layer workflow, which adds the pinia module and registers some stores in the base layer. I want those stores to be available to my containing app, as well as stores that the containing app defines.

The issues I found with the current state of the module include:

1. The pinia module tries to resolve pinia/dist/pinia.mjs from the containing app. This is problematic, because the layer is the one bringing the pinia dependency in. It should be using the distributable that the layer depends on.
2. The stores directory in the layer gets lost when the containing app tries to register its own stores.
3. Extending the layer should bring in the stores from the layer, and allow adding stores from the containing app.

I believe this PR solves those three problems. We remove some use of a deprecated API as well.

To add tests to this, I think I'd need to create a new "playground-layer" directory and extend from that in a new nuxt spec pertaining to layer functionality. If you agree with these changes, we can discuss if you'd like that as part of this.

Note: This may break nuxt 2 compatibility. I can try to add back support if this is a decent enough approach to fixing layer support.

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