Revert "Use a less verbose package name"

This reverts commit 9a950978f04cfae7a72f6e7de4f4dca89b1d607a.
The package name is already registered at packagist.org. The original
intent was to rename the default module directory from
"silverstripe-autotoc" to "autotoc" but this seems to be the wrong way.
Improve composer.json compatibility

Follow the hints provided by SilverStripe developers at
http://addons.silverstripe.org/submit

Furthermore, trying to change the default installation directory by
defining the somewhat undocumented extra/installer-name option.