Issue #2380389 by webflo, Mile23, davidwbarratt, timmillwood, hussainweb, tstoeckler, bojanz, yched, MKorostoff, alexpott, webchick: Use a single vendor directory in the root

Issue #1959660 by larowlan, pfrenssen, xjm, clemens.tolboom, dawehner, RobLoach: Replace xpath() with WebTestBase::cssSelect() by leveraging Symfony CssSelector.