Reverse direction of dependencies when determining target packages.

Fixes googleapis#3466.

test_utils/scripts/get_target_packages.py

@@ -44,6 +44,11 @@
     'speech': {'storage'},
     'vision': {'storage'},
 }
+REVERSE_DEPS = {}
+for pkg, deps in PKG_DEPENDENCIES.items():
+    for dep in deps:
+        REVERSE_DEPS.setdefault(dep, set()).add(pkg)
+del pkg, deps
 
 
 def get_baseline():
@@ -147,7 +152,7 @@ def get_changed_packages(file_list):
         # Add the package, as well as any dependencies this package has.
         # NOTE: For now, dependencies only go down one level.
         answer.add(package)
-        answer = answer.union(PKG_DEPENDENCIES.get(package, set()))
+        answer = answer.union(REVERSE_DEPS.get(package, set()))
 
     # We got this far without being short-circuited; return the final answer.
     return answer