Refactor InlineModelResolver (OpenAPITools#1788)

* Extract a method "flattenPaths" to reduce the scope of method

* Tweak

* Rename parameter name

* Extract a method "flattenModels" to reduce the scope of method

* Rename parameter name

* Rename: models -> components

* Delete comment

* Extract a method "flattenRequestBody" to reduce the scope of method

* Extract a method "flattenParameters" to reduce the scope of method

* Extract a method "flattenResponses" to reduce the scope of method

* Tweak types

* Reduce indentation