Fix "method_complexity" issue in Sources/AssetLib/Controllers/ImageSetTemplateBuilder.swift

[leogdion]

Function document has a Cognitive Complexity of 13 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/brightdigit/AssetLib/Sources/AssetLib/Controllers/ImageSetTemplateBuilder.swift#issue_5e74b328b3518f0001000083