Add `graphile-build-pg` as an explicit dependency

[benjie]

It's used here:

postgraphile-plugin-connection-filter/src/PgConnectionArgFilterPlugin.js

Line 1 in fc2d94f

const { omit } = require("graphile-build-pg");

The omit method doesn't need to come from the same version of graphile-build-pg as the schema is built with, so it doesn't matter if this causes a duplicate (although hopefully it won't!)

Reason this came to my attention:

graphile/crystal#778

In future, I should add it to the build object: graphile/graphile-engine#235

[mattbretl]

The explicit dep is now present in v1.0.0-beta.11.