Handle Rubyland dependency resolution

[demsullivan]

The goal of this PR is to establish core support for dry-types/dry-struct dependency resolution in Rubyland.

What this means is that dry-typescript needs to do its best to identify and follow references from a type to another type, possibly in another module.

A simple example:

module Types
  include Dry.Types()
  extend Dry::Typescript

  UUID = Types::String
  # In this line, the UUID constant is being referenced. dry-typescript needs
  # to know about this and track it. Since types aren't classes or modules, they don't have a
  # `.name` method to work with.
  ts_export User = Types::Hash.schema(id: UUID)

  finalize_ts_exports!

A more complex example:

module Types
  include Dry.Types()

  UUID = Types::String
  User = Types::Hash.schema(id: UUID)
end

module TypeExports
  extend Dry::Typescript

  # Types::User is being referenced here. dry-typescript needs to know about
  # and track this dependency on Types::User, but also the subsequent dependency
  # on Types::UUID, in order to export the correct type aliases.
  ts_export Users = Types::Array.of(Types::User)

Fixes #10
Fixes #13