Interfaces duplicates for `single` grouping option

[krodnar]

Nature of issue?

• Existing feature enhancement
• New feature request

Issue description

Currently with a single file option for grouping we are getting a big file with a bunch of interfaces just thrown out there. In this case usually we get a lot of duplicates and broken interfaces.

Solution

There are 3 options of dealing with this issue that I came up with.

First and the easiest one is to remove single file option completely.
Second and the hardest one is to make some smart duplicates remover.
And the third one is to declare every interface in it's own module based on endpoint group, for example.