Fix overwrite bug in $file parameter

Previously $file, now $_fi1e_xYzZy  - given an unlikely name so that extract does not overwrite it.

See http://www.concrete5.org/developers/bugs/5-6-1-2/loaderpackageelement-breaks-on-file-as-parameter/

Similar bugs could occur for other variables such as $args or $env, both likely names for parameters to elements that could be crapped on by extract(). However, I will see what response this fix gets before doing more work on such bugs.