Ft/handle external pkg name in response

[masu-mi]

Overview

an example resolving of https://github.com/deepmap/oapi-codegen/issues/751

changes

• add test internal/test/issues/issue-removed-external-ref
• add UppercaseFirstCharacterWithPkgName function for ucFirstWithPkgName filter
• use ucFirstWithPkgName filter instead of ucFirst filter in template