Added VisitDirectDeps* methods to SingletonContext.
Added VisitDirectDeps* methods to SingletonContext.

A an example use case is generating the graph of module
dependencies, where modules are vertices and dependencies
are edges.

The patch was tested with a local implementation of such
a module in the Soong build system. The graph was generated
for all modules in the AOSP tree.

google/blueprint,danw/blueprint,danw/blueprint,google/blueprint