Improved the performance by refactoring the $app->edition() method

The `edition()` method no longer calls internally the `book()` method.

Xdebug profiling shows that after this change in the `edition()` method,
when publishing a sample book the `$app->book()` method was called
around 70 times instead of the original 400 times.

The benchmark command also confirms a performance improvement of nearly
2 points.
Removed some duplicated code in the publisher classes

This error was detected by the PHP Copy/Paste Detector of Scrutinizer CI
see https://scrutinizer-ci.com/g/javiereguiluz/easybook/inspections/4adf3d92-b89f-4ce7-921e-2149429fb476/issues#inspectioncomment-11436431