_codegen: add license and merge package 'imports'

[dolmen]

Summary

Merge code from package internal/imports (recently copied from package github.com/ernesto-jimenez/gogen/imports) into the codegen program itself. Also copy the license attached to it into the _codegen/main.go source, as _codegen/main.go was originally by the same author (@ernesto-jimenez) as the imported code and matches the license terms of the global Testify project. Also: @ernesto-jimenez who was queried about this project, didn't block it. https://github.com/stretchr/testify/pull/1782#discussion_r2322798422

Changes

• add license from _codegen/internal/imports/imports.go into _codegen/main.go, and add also other _codegen contributor (from Git history)
• copy type _codegen/internal/imports.imp into _codegen/main.go as imports.
• remove package _codegen/internal/imports that isn't need anymore

Motivation

Reduce code in _codegen

Related issues

This a follow-up to #1782.