Take proper care with the order of operations when replacing tasks.

This logic is fragile and should probably be removed entirely.

Testing Done:
Manually verified that this fixes foursquare.web's issues with replacing all tasks in a goal.

ci running.

Reviewed at https://rbcommons.com/s/twitter/r/1221/

src/python/pants/goal/goal.py

@@ -128,15 +128,13 @@ def install(self, task_registrar, first=False, replace=False, before=None, after
     subclass_name = b'{0}_{1}'.format(task_registrar.task_type.__name__,
                                       options_scope.replace('.', '_').replace('-', '_'))
     task_type = type(subclass_name, (task_registrar.task_type,), {'options_scope': options_scope})
-    if replace:
-      self._task_type_by_name = {}
-    self._task_type_by_name[task_name] = task_type
 
     otn = self._ordered_task_names
     if replace:
-      for task_type in self.task_types():
-        task_type.options_scope = None
+      for tt in self.task_types():
+        tt.options_scope = None
       del otn[:]
+      self._task_type_by_name = {}
     if first:
       otn.insert(0, task_name)
     elif before in otn:
@@ -146,6 +144,7 @@ def install(self, task_registrar, first=False, replace=False, before=None, after
     else:
       otn.append(task_name)
 
+    self._task_type_by_name[task_name] = task_type
     self.dependencies.update(task_registrar.dependencies)
 
     if task_registrar.serialize: