Fix self-type substitution for reopened generic modules

[marcoroth]

This pull request fixes module self-type validation when a generic module is reopened with different type parameter names.

RBS treats the type parameters of reopened declarations positionally, so declarations using E and Elem are equivalent.

Steep previously substituted every self type using the parameter names from the module's primary declaration. A self type originating from another declaration could therefore retain an unresolved type variable and incorrectly produce RBS::ModuleSelfTypeError.

The signature validator now finds the declaration that introduced each self type and uses that declaration's parameter names for substitution. A regression test covers equivalent module reopenings with differently named parameters:

module Enum[E] : _Each[E]
end

module Enum[Elem] : _Each[Elem]
end

class Collection
  include Enum[String]

  def each: () { (String) -> void } -> void
end

This also allows the RBS 4.1.2 core and shim declarations for Enumerable to be validated together.

Resolves #2254