Moved internal headers into the intern/ subdirectory.

The depsgraph folder itself with the main API headers can then be made
available to other Blender parts while the internal headers are hidden.
Renamed the BKE_ headers inside depsgraph to DEG_ prefix.

This code is node actually included in blenkernel. Using unique prefixes
also helps porting code to the new depsgraph without include name
clashes.
Fixed include guard defines to use the new DEG prefix.
Missing struct tag for undefined ID struct.
Stub implementations to avoid compiler error.
Initialize and free the depsgraph node typeinfo on Blender start/exit.
Proxy node type was using wrong type identifier.