Add transitive parameter to UnionProducts.get_for_target[s]

Testing Done:
CI is baking: https://travis-ci.org/pantsbuild/pants/builds/78112646

Added tests for the new parameter to:
  * `tests/python/pants_test/goal/test_union_products.py:test_get_for_target`.

Reviewed at https://rbcommons.com/s/twitter/r/2741/

src/python/pants/goal/products.py

@@ -38,17 +38,21 @@ def remove_for_target(self, target, products):
     for product in products:
       self._products_by_target[target].discard(product)
 
-  def get_for_target(self, target):
+  def get_for_target(self, target, transitive=True):
     """Gets the transitive product deps for the given target."""
-    return self.get_for_targets([target])
+    return self.get_for_targets([target], transitive=transitive)
 
-  def get_for_targets(self, targets):
+  def get_for_targets(self, targets, transitive=True):
     """Gets the transitive product deps for the given targets, in order."""
     products = OrderedSet()
     visited = set()
     # Walk the targets transitively to aggregate their products. We do a breadth-first
     for target in targets:
-      for dep in target.closure(bfs=True):
+      if transitive:
+        deps = target.closure(bfs=True)
+      else:
+        deps = [target]
+      for dep in deps:
         if dep not in visited:
           products.update(self._products_by_target[dep])
           visited.add(dep)

tests/python/pants_test/goal/test_union_products.py

@@ -27,6 +27,9 @@ def test_get_for_target(self):
     self.assertEquals(self.products.get_for_target(a), OrderedSet([1, 2, 3]))
     self.assertEquals(self.products.get_for_target(b), OrderedSet([2, 3]))
     self.assertEquals(self.products.get_for_target(c), OrderedSet([3]))
+    self.assertEquals(self.products.get_for_target(a, transitive=False), OrderedSet([1]))
+    self.assertEquals(self.products.get_for_target(b, transitive=False), OrderedSet([2]))
+    self.assertEquals(self.products.get_for_target(c, transitive=False), OrderedSet([3]))
 
   def test_remove_for_target(self):
     c = self.make_target('c')