Adds a new external-ref-named-imports option which changes aliases for external imports from externalRef0, externalRef1, ... to external_ref_<package_name> (e.g. external_ref_packageA, external_ref_packageB).
This allows for more consistent import names when dealing with large projects and multiple linked specs, where the same externalRef0 import can end up referencing different source packages.

Motivation

I'm using oapi-codegen in a complex project in combination with tygo to generate TS types for my frontend code. tygo also supports external imports, but they need to be explicitly mapped from the Go package to the TS file, so my config for that currently looks something like this:

type_mappings:
  time.Time: "string /* RFC3339 */"
  null.String: "null | string"
  null.Bool: "null | boolean"
  uuid.UUID: "string /* uuid */"

  externalRef0.User: User
  externalRef1.Project: Project
  externalRef1.Repo: Repo
  externalRef2.Identity: Identity
  // same source package but different ref index for a spec that doesn't import User or Project, leading to duplicate type mapping
  externalRef0.Identity: Identity 

needing to map the same package/type multiple times since different files have varying starting indexes for externalRef imports.
Using named-imports allows me to simply do this instead:

  external_ref_users.User: User
  external_ref_projects.Project: Project
  external_ref_projects.Repo: Repo
  external_ref_identities.Identity: Identity