Adds support for specifying a package alias for external imports, which allows using custom names instead of auto-generated aliases like externalRef0, externalRef1, ...
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 the custom alias allows me to simply do this instead:

# config.yaml
import-mapping:
  ./common/users.yaml: users example_package/users
  ./common/projects.yaml: projects example_package/projects
  ./common/identities.yaml: identities example_package/identities

# tygo.yaml
  users.User: User
  projects.Project: Project
  projects.Repo: Repo
  identities.Identity: Identity

previous implementation: #2193