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
diff --git a/Android.bp b/Android.bp
index 94b4f7e..c0a1aeb 100644
--- a/Android.bp
+++ b/Android.bp
@@ -56,6 +56,7 @@
         "android/neverallow.go",
         "android/onceper.go",
         "android/package_ctx.go",
+        "android/path_properties.go",
         "android/paths.go",
         "android/prebuilt.go",
         "android/prebuilt_etc.go",
@@ -80,6 +81,7 @@
         "android/namespace_test.go",
         "android/neverallow_test.go",
         "android/onceper_test.go",
+        "android/path_properties_test.go",
         "android/paths_test.go",
         "android/prebuilt_test.go",
         "android/prebuilt_etc_test.go",