Fix issue with autopkg (URLTextSearcher) referencing output variables during processing.

Fixes an issue with autopkg, specifically here:

https://github.com/autopkg/autopkg/blob/master/Code/autopkglib/__init__.py#L401

.. where autopkg is expecting to reference the output variables. So we just dynamically generate the `output_variables` dictionary during execution which works as `output_variables` is references after this processing.