do away with non-conventional "common" module

closes FOO-2459
flag=none

move defined constants into the sis module proper

TEST PLAN:
  1) specs pass

Change-Id: I053e551055a65b7de2a0c23cd571599dc4a8d64f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/275450
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

lib/sis.rb

@@ -19,4 +19,5 @@
 #
 
 module SIS
+  class ImportError < StandardError; end
 end

lib/sis/base_importer.rb

@@ -18,7 +18,7 @@
 # with this program. If not, see <http://www.gnu.org/licenses/>.
 #
 
-require_dependency 'sis/common'
+require_dependency 'sis'
 
 module SIS
   class BaseImporter

lib/sis/common.rb

@@ -1,23 +0,0 @@
-# frozen_string_literal: true
-
-#
-# Copyright (C) 2011 - present Instructure, Inc.
-#
-# This file is part of Canvas.
-#
-# Canvas is free software: you can redistribute it and/or modify it under
-# the terms of the GNU Affero General Public License as published by the Free
-# Software Foundation, version 3 of the License.
-#
-# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
-# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
-# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
-# details.
-#
-# You should have received a copy of the GNU Affero General Public License along
-# with this program. If not, see <http://www.gnu.org/licenses/>.
-#
-
-module SIS
-  class ImportError < StandardError; end
-end

lib/sis/csv/csv_base_importer.rb

@@ -20,7 +20,7 @@
 
 require 'csv'
 
-require_dependency 'sis/common'
+require_dependency 'sis'
 
 module SIS
   module CSV