Rework the console path

[theofidry]

Outside of a Symfony app the bin/console path does not make sense. Most of the time however, the script path remains unchanged between the main and child processes so we can make use of that instead.

This change means however that during the tests script path is no longer valid (it is a problem only for child processes) but this can easily be solved by explicitly declaring the script path.

Besides this change:

• Rename the getConsolePath() to getScriptPath() which is a more generic name which is more fitting when working outside of a Symfony app
• Move the validation of the script path outside of ParallelExecutor