Depsgraph: Comb code to a better state all over

Some summary of changes:

- Don't use DEG prefix for types and enumerator values:
  the code is already inside DEG namespace.

- Put code where it locally belongs to: avoid having one
  single header file with all sort of definitions in it.

- Take advantage of modern C++11 enabled by default.

source/blender/depsgraph/intern/depsgraph_physics.h

@@ -0,0 +1,46 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2019 Blender Foundation.
+ * All rights reserved.
+ *
+ * Original Author: Sergey Sharybin
+ * Contributor(s): None Yet
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/depsgraph/intern/depsgraph_physics.h
+ *  \ingroup depsgraph
+ */
+
+#pragma once
+
+struct Collection;
+struct ListBase;
+
+namespace DEG {
+
+struct Depsgraph;
+
+ListBase *build_effector_relations(Depsgraph *graph, Collection *collection);
+ListBase *build_collision_relations(Depsgraph *graph,
+                                    Collection *collection,
+                                    unsigned int modifier_type);
+void clear_physics_relations(Depsgraph *graph);
+
+}  // namespace DEG