Cleanup: Rename file used for calculating mesh edges

This commit renames `mesh_validate.cc` to `mesh_calc_edges.cc`.
I would like to move `mesh_validate.c` to C++, but it makes sense to
keep this specific algorithm in a smaller file.

source/blender/blenkernel/CMakeLists.txt

@@ -183,6 +183,7 @@ set(SRC
   intern/mball_tessellate.c
   intern/mesh.cc
   intern/mesh_boolean_convert.cc
+  intern/mesh_calc_edges.cc
   intern/mesh_convert.cc
   intern/mesh_debug.cc
   intern/mesh_evaluate.cc
@@ -199,7 +200,6 @@ set(SRC
   intern/mesh_tangent.c
   intern/mesh_tessellate.c
   intern/mesh_validate.c
-  intern/mesh_validate.cc
   intern/mesh_wrapper.c
   intern/modifier.c
   intern/movieclip.c

source/blender/blenkernel/intern/mesh_calc_edges.cc

@@ -7,10 +7,7 @@
 #include "DNA_mesh_types.h"
 #include "DNA_meshdata_types.h"
 #include "DNA_object_types.h"
-
-#include "BLI_edgehash.h"
 #include "BLI_map.hh"
-#include "BLI_math_base.h"
 #include "BLI_task.hh"
 #include "BLI_threads.h"
 #include "BLI_timeit.hh"