Move relative position range related classes to own files

In development some of theese classes are not autoloaded, so
we are better off moving these to separate files instead.

lib/gitlab/relative_positioning/closed_range.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module RelativePositioning
+    class ClosedRange < RelativePositioning::Range
+      def initialize(lhs, rhs)
+        @lhs, @rhs = lhs, rhs
+        raise IllegalRange, 'Either lhs or rhs is missing' unless lhs && rhs
+        raise IllegalRange, 'lhs and rhs cannot be the same object' if lhs == rhs
+      end
+    end
+  end
+end

lib/gitlab/relative_positioning/ending_at.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module RelativePositioning
+    class EndingAt < RelativePositioning::Range
+      include Gitlab::Utils::StrongMemoize
+
+      def initialize(rhs)
+        @rhs = rhs
+        raise IllegalRange, 'rhs is required' unless rhs
+      end
+
+      def lhs
+        strong_memoize(:lhs) { rhs.lhs_neighbour }
+      end
+    end
+  end
+end

lib/gitlab/relative_positioning/range.rb

@@ -31,39 +31,5 @@ module Gitlab
         other.is_a?(RelativePositioning::Range) && lhs == other.lhs && rhs == other.rhs
       end
     end
-
-    class ClosedRange < RelativePositioning::Range
-      def initialize(lhs, rhs)
-        @lhs, @rhs = lhs, rhs
-        raise IllegalRange, 'Either lhs or rhs is missing' unless lhs && rhs
-        raise IllegalRange, 'lhs and rhs cannot be the same object' if lhs == rhs
-      end
-    end
-
-    class StartingFrom < RelativePositioning::Range
-      include Gitlab::Utils::StrongMemoize
-
-      def initialize(lhs)
-        @lhs = lhs
-        raise IllegalRange, 'lhs is required' unless lhs
-      end
-
-      def rhs
-        strong_memoize(:rhs) { lhs.rhs_neighbour }
-      end
-    end
-
-    class EndingAt < RelativePositioning::Range
-      include Gitlab::Utils::StrongMemoize
-
-      def initialize(rhs)
-        @rhs = rhs
-        raise IllegalRange, 'rhs is required' unless rhs
-      end
-
-      def lhs
-        strong_memoize(:lhs) { rhs.lhs_neighbour }
-      end
-    end
   end
 end

lib/gitlab/relative_positioning/starting_from.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module RelativePositioning
+    class StartingFrom < RelativePositioning::Range
+      include Gitlab::Utils::StrongMemoize
+
+      def initialize(lhs)
+        @lhs = lhs
+        raise IllegalRange, 'lhs is required' unless lhs
+      end
+
+      def rhs
+        strong_memoize(:rhs) { lhs.rhs_neighbour }
+      end
+    end
+  end
+end