Prefer existing project nodes to external nodes, even if the project node is disabled

[MichelleArk]

Housekeeping

• [X] I am a maintainer of dbt-core

Short description

Currently, an external node can clobber an existing project node if that project node is disabled. This should not be the case as it leads to duplicative nodes in the manifest! (one in manifest.nodes, one in manifest.disabled)

Acceptance criteria

When injecting an external node to the manifest that already exists from the FS, handle merges by:

1. Enabled project nodes are preferred to external nodes
2. Disabled project nodes are preferred to external nodes

Suggested Tests

tests follow from A/C :)

Impact to Other Teams

N/A

Will backports be required?

nope

Context

No response