Eagerly define attribute methods for internal models

These models no longer override `table_exists?` to bypass the schema
cache since c5ecc33.

activerecord/lib/active_record/core.rb

@@ -427,10 +427,6 @@ def type_caster # :nodoc:
         TypeCaster::Map.new(self)
       end
 
-      def _internal? # :nodoc:
-        false
-      end
-
       def cached_find_by_statement(key, &block) # :nodoc:
         cache = @find_by_statement_cache[connection.prepared_statements]
         cache.compute_if_absent(key) { StatementCache.create(connection, &block) }

activerecord/lib/active_record/internal_metadata.rb

@@ -15,10 +15,6 @@ def enabled?
         ActiveRecord::Base.connection.use_metadata_table?
       end
 
-      def _internal?
-        true
-      end
-
       def primary_key
         "key"
       end

activerecord/lib/active_record/railtie.rb

@@ -166,10 +166,6 @@ class Railtie < Rails::Railtie # :nodoc:
           if app.config.eager_load
             begin
               descendants.each do |model|
-                # SchemaMigration and InternalMetadata both override `table_exists?`
-                # to bypass the schema cache, so skip them to avoid the extra queries.
-                next if model._internal?
-
                 # If the schema cache was loaded from a dump, we can use it without connecting
                 schema_cache = model.connection_pool.schema_cache
 

activerecord/lib/active_record/schema_migration.rb

@@ -10,10 +10,6 @@ module ActiveRecord
   # to be executed the next time.
   class SchemaMigration < ActiveRecord::Base # :nodoc:
     class << self
-      def _internal?
-        true
-      end
-
       def primary_key
         "version"
       end