Use WalkDeps instead of VisitDepsDepthFirst

VisitDepsDepthFirst is almost never correct, as it can't query
dependency tags of multiple dependencies between the same two modules.
Replace VisitDepsDepthFirst with WalkDeps in sanitize.go and
python.go.  Also verify the dependency tag before continuing to
recurse to ensure that they don't recurse through genrules and into
unrelated modules.

Test: m checkbuild
Change-Id: I2f7560126f56b51a40ec39dfbdcc18b5891489f7