Class DirBuildInfo

   object --+    
            |    
BuildInfoBase --+
                |
               DirBuildInfo

The generic base class for build information for a Node.

This is what gets stored in a .sconsign file for each target file.
It contains a NodeInfo instance for this node (signature information
that's specific to the type of Node) and direct attributes for the
generic build stuff we have to track:  sources, explicit dependencies,
implicit dependencies, and action information.