Use include directive to specify cop directories

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@.luke made the following suggestion in this MR:

In .rubocop.yml you can define which directories to apply the cop, using the Include directive. Although we don't use this as much as we should, we could set it to only inspect the relevant graphql directories. I think this would speed up our cop inspections just a little bit when people are working outside of graphql.

We should refactor some of the existing cops to do directory filtering via config, not programmatically.