Issue #2737773 by antongp, wturrell, pcambra, cilefen, Darvanen, cwells, manningpete, alexpott: Proper way to install Drupal, missing vendor folders, example.gitignore

Issue #1475510 by timmillwood, hussainweb, davidwbarratt, alexpott, Mile23, pfrenssen, claudiu.cristea, webchick, catch, Crell, patcon, RobLoach, kim.pepper, dawehner, neclimdul, Mixologic, gagarine, sun, nod_, effulgentsia, drumm, Sylvain Lecoy, jhedstrom, katbailey, xjm, klausi, Seldaek, Barryvdh: Remove external dependencies from the core repo and let Composer manage the dependencies instead

Issue #1975220 by davidwbarratt, tstoeckler, mradcliffe, RobLoach, Mile23, hussainweb: Allow a Composer user to manage Drupal, modules, and PHP dependencies with a custom root composer.json