Fix 'already initialized constant' warning

Warning in lib/backup/files occurred when running specs locally:
warning: already initialized constant Backup::Files::DEFAULT_EXCLUDE

Fix is consistent with solution in
https://docs.gitlab.com/ee/development/gotchas.html#solution-1

lib/backup/artifacts.rb

 # frozen_string_literal: true
 
-require 'backup/files'
-
 module Backup
-  class Artifacts < Files
+  class Artifacts < Backup::Files
     attr_reader :progress
 
     def initialize(progress)

lib/backup/builds.rb

 # frozen_string_literal: true
 
-require 'backup/files'
-
 module Backup
-  class Builds < Files
+  class Builds < Backup::Files
     attr_reader :progress
 
     def initialize(progress)

lib/backup/lfs.rb

 # frozen_string_literal: true
 
-require 'backup/files'
-
 module Backup
-  class Lfs < Files
+  class Lfs < Backup::Files
     attr_reader :progress
 
     def initialize(progress)

lib/backup/pages.rb

 # frozen_string_literal: true
 
-require 'backup/files'
-
 module Backup
-  class Pages < Files
+  class Pages < Backup::Files
     attr_reader :progress
 
     def initialize(progress)

lib/backup/registry.rb

 # frozen_string_literal: true
 
-require 'backup/files'
-
 module Backup
-  class Registry < Files
+  class Registry < Backup::Files
     attr_reader :progress
 
     def initialize(progress)

lib/backup/uploads.rb

 # frozen_string_literal: true
 
-require 'backup/files'
-
 module Backup
-  class Uploads < Files
+  class Uploads < Backup::Files
     attr_reader :progress
 
     def initialize(progress)