(snap-backed-graphql-7) Back all dependency-related things via snap
This uses the pipeline and dependency indexes to back all
dependency-related graphql types.
This is the most thorny part of the graphql schema and I believe it
should be relatively straightforward from here to convert the rest
of the schema to be snapshot backed.
Depends on D2310
Test Plan: BK
Reviewers: alangenfeld, sashank, themissinghlink
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D2311