Speed up isDependency query when spec depPkg has pkgID (add ent Pg index)

[nchelluri]

Description of the PR

I found that doing isDependency GQL queries where the filter had a dependent package spec with the package ID specified was slow, and adding this index sped it up a fair bit.

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If ent schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged