Fix associations altered by another association

[groue]

This pull request fixes some requests that involve an association altered by another association.

For example, the Author.translatedBooks association below is such an altered association:

extension Book {
    static let translator = belongsTo(Translator.self)
}
extension Author {
    static let books = hasMany(Book.self)
    
    // A book is translated if it is associated to a translator
    static let translatedBooks = books.joining(required: Book.translator)
}

Some requests that involve those altered associations used to erroneously drop the alteration.

• Requests which use the altered association with the request(for:) method:

  // Fixed
  extension Author {
      var translatedBooks: QueryInterfaceRequest<Book> {
          request(for: Author.translatedBooks) // !
      }
  }
  let author: Author = ...
  let translatedBooks: [Book] = try author.translatedBooks.fetchAll(db)

• Requests that eager load an hasManyThrough association based on the altered association:

  // Fixed
  extension Book {
      static let awards = hasMany(Award.self)
  }
  extension Author {
      static let awardsForTranslatedBooks = hasMany(
          Award.self,
          through: translatedBooks,            // !
          using: Book.awards)
  }
  struct AuthorInfo: Decodable, FetchableRecord {
      var author: Author
      var awardsForTranslatedBooks: [Award]
  }
  let authorInfos: [AuthorInfo] = try Author
      .including(all: Author.awardsForTranslatedBooks)
      .asRequest(of: AuthorInfo.self)
      .fetchAll(db)