[entropy.client.package] synchronize text output calls to avoid garbled output

Some parts of the package actions code are now multithreaded and some
other will likely become that in the near future.
For this reason, all the TextInterface.output calls are now wrapped and
synchronized by a mutex.

Ideally, this synchronization could have gone directly into entropy.output
but I am reluctant about placing expensive locking so deep into the code at
this time.