Allow default packages

[BrianHenryIE]

If the "packages" config for mozart under composer's extras field is absent, use the full list of composer's required packages.

[ChristophWurst]

👍 Makes sense

[coenjacobs]

Sorry to leave this hanging for so long, it makes perfect sense. Thanks for your contribution, @BrianHenryIE!

[BrianHenryIE]

This merge did not work. On line 50 array_keys(get_object_vars($this->config->require)), $this->config is the Mozart extra object, but it needs to be the full composer.json object whose require key is to be used. The PR as is works, so can you re-merge, or should I create a new one?

I also think the use of $composer = in the PR is clearer than using $config twice to mean two different things.