SERVER-9670 Use LIBDEPS to declare dependents in addition to prerequisites.

To achieve this, the "direct prerequisites" for a File node are stored in the
node's attributes object, instead of in the build environment for the node.
This allows the emitter for nodes to amend _other nodes_ prerequisites.  After that,
this change is trivial.  Introduce a LIBDEPS_DEPENDENTS environment variable that sets
the named dependents of a node, while the existing LIBDEPS variable may be considered
the direct prerequisites.