[CT-2709] Generalize manifest.public_node

[MichelleArk]

The initial implementation of resolving refs to models from upstream projects significantly special-cased the storage and handling of "upstream public nodes". It led to some unnecessary duplication to achieve the same behaviours that model nodes defined within a project had -- a node that can be referenced, selected (via list / --select syntax) and executed (depending on its presence of raw code).

This can be reconciled by handling upstream public nodes much like dbt currently handles models from upstream packages to avoid duplication and make the implementation more straightforward to reason about. manifest.nodes already stores a collection of generic nodes that are part of dbt's DAG -- including those defined by other projects (i.e. packages), and should be reused to store public nodes (as ModelNodes), injected at the end of parsing.