There was a function in swift.common.utils that was importing
swob.HeaderKeyDict at call time. It couldn't import it at compilation
time since utils can't import from swob or else it blows up with a
circular import error.
This commit just moves HeaderKeyDict into swift.common.header_key_dict
so that we can remove the inline import.
Change-Id: I656fde8cc2e125327c26c589cf1045cb81ffc7e5