also export channels used by pre populated fields

[aleppax]

when exporting a channel, include also channels used by fields which populate their list of options with them This kind of dependency is similar to a relationship and is helpful to export what is needed to preserve the channel integrity.

Nature of This Change

Is this backwards compatible?

• [ ] Yes
• [ ] No

Documentation

User Guide Pull Request: https://github.com/ExpressionEngine/ExpressionEngine-User-Guide/pull/NNN

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