As Composer is now widely used in the PHP world, having to run composer
install before running the test suite is expected. This also has the
nice benefit of removing a bunch of code, making things easier to
maintain (there is only one place to declare a dev dependency), and
probably more.
The Config component is a hard dependency for the loaders (but loaders
themselves are optional); all other classes should not have a hard dep
on Config. The introduction of a new flag allows to remove this
dependency.
This commit also fixes skipped test dependencies.