Fix detection of pgSelect allowing merging

[benjie]

Description

#1987 was only an issue because the function wasn't optimized and was being called via a second SQL query. This was undesirable. It turns out to be due to the dependency checking in pgSelect's optimize method. Really we just want to know that the dependencies can be pushed onto the new parent... so I've rewritten the code to do this instead.

Performance impact

Significant improvement in some cases.

Security impact

Helps avoid DOS I guess?

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[changeset-bot]

🦋 Changeset detected

Latest commit: c48d3da

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 15 packages

Name	Type
grafast	Patch
postgraphile	Patch
@dataplan/pg	Patch
@localrepo/grafast-bench	Patch
@dataplan/json	Patch
@grafserv/persisted	Patch
grafserv	Patch
ruru-components	Patch
@localrepo/grafast-website	Patch
graphile-build-pg	Patch
graphile-build	Patch
graphile-utils	Patch
pgl	Patch
graphile-export	Patch
graphile	Patch

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