This entirely replaces our custom autoload mechanism. The added benefit
is reduced maintenance for DomPDF itself but also its dependencies. And
it opens up the possibility to add new dependency transparently, without
the need to manage their autoloading.

Fixes #2608
Closes #2779
Fixes https://github.com/dompdf/utils/issues/12
Closes https://github.com/dompdf/utils/pull/13