[codegen] add additionalImportedModulesNames to apollo codegen config

[hemel]

The purpose of this change is to give codegen the capability to add custom import statements to generated operations.
This change would enable the consumer to share fragments across generated packages.

Addresses this thread:

apollographql/apollo-ios#3284

[calvincestari]

Thanks for the PR @hemel but I don't think this correctly solves the issue in apollographql/apollo-ios#3284.

The problem description is three modules (A, B, C) and only two (A, B) need to share the fragments (D); C should not import D. Your solution essentially replicates putting the fragments into the shared schema module which all operation files already import.

I'm going to close this PR but if you want to keep iterating on it please go ahead. Manipulating the templates is the easy part but finding an elegant configuration model is the difficult part.

[jimisaacs]

@hemel I'm confused, these don't look like all the changes we landed on? Can you share some context?

[jimisaacs]

@hemel nevermind, was confused found #158