Fix KeyError from defer + deletion

[jtcohen6]

resolves #2875

Description

The fix for this is a lot simpler than I'd previously thought.

Because of --defer, it's possible that the --state manifest has recorded some upstream nodes (in depends_on) that no longer exist in the current project. When that upstream node is a source or ephemeral model, it raised a KeyError. The fix just required that, before we try to record an edge, we first check that the node being reported by the other manifest actually exists in our known set of project nodes.

I wrote out one test case, run_defer_deleted_upstream, which involved an upstream ephemeral model that exists in the deferral manifest but not in the current project. I confirmed that this test raises a KeyError without the one-line fix, and passes with it.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[kwigley]

👏 nice test!