Add path properties mutator

Add a mutator pass after DepsMutator that visits every property
struct in every module looking for properties that have a tag
`android:"path"`, and automatically add a SourceDepTag dependency
on any module references (":module-name") found.  Uses a cache to
store the mapping of property struct type to locations of
properties with the tag.

Test: android/path_properties_test.go
Change-Id: I38c0497843dde4890e9342c3a6f0b402c0720742