Prevent duplicate visit calls in WalkDeps

WalkDeps was following every possible path through the dependency
tree, which can be enormous.  Modify it to only call visit for
any particular (child, parent) pair once for each direct dependency
by not recursing into child if visitDown returns true but child
has already been visited.

Test: TestWalkDeps, TestWalkDepsDuplicates
Change-Id: Ieef28399bd10e744417cdeb661dfa04fbeb4ec60