Issue #2405241 by philipnorton42, jibran, dawehner, andrewmacpherson: Add helpful exception message in CustomAccessCheck

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

(cherry picked from commit 96f86e68)

Issue #2331079 by znerol, dawehner: Use RouteMatch in access-checks and remove RequestHelper::duplicate().

Issue #2222719 by effulgentsia, tim.plunkett, xjm, dawehner: Use parameter matching via reflection for access checks instead of pulling variables from request attributes.

Issue #2109035 by tim.plunkett, damiankloip, dawehner, chx: Make access checkers (much) easier to find.

Issue #2048223 by dawehner, ParisLiakos, herom, fubhy, damiankloip, vijaycs85, joelpittet, tim.plunkett: Add $account argument to AccessCheckInterface::access() method and use the current_user() service.

Issue #2107137 by dawehner, damiankloip, tim.plunkett: Fix the DX for declaring custom access checkers.