Issue #2763157 by tim.plunkett, tedbow, chx, bojanz, twistor, neclimdul, dawehner, Xano, EclipseGc: Allow plugins to provide multiple forms

Revert "Issue #2773197 by jalpesh, tstoeckler: DefaultHtmlRouteProviderTest::testGetCollectionRoute() has wrong @covers"

This reverts commit a470bc0c.

Issue #2773197 by jalpesh, tstoeckler: DefaultHtmlRouteProviderTest::testGetCollectionRoute() has wrong @covers

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

Issue #1850080 by amateescu, dawehner, Xano, bojanz, martin107, Pancho, alexpott, tim.plunkett, icseh., stefank, Gábor Hojtsy, jhodgdon, tstoeckler: Entity type labels lack plurality, cannot generate UI text based on label if plural is needed

Issue #1392754 by tss, justAChris, jhodgdon: Comply with new documentation standards for @file for namespaced class files

Issue #2443409 by Berdir: Add a way to entity manager to get fresh entity and field definitions without invalidating all caches

Issue #2328111 by dawehner, martin107, neclimdul: Replace most instances of the DRUPAL_ROOT constant with the app.root container parameter.

Issue #2340123 by Wim Leers: Setting cache tags can be tricky: use strings instead of nested arrays to improve DX.

Issue #2318281 by tim.plunkett, dawehner | kim.pepper: Provide a standard way to check a plugin is an instance of the required interface.

Issue #2270323 by sun, dawehner, neclimdul: Remove DRUPAL_ROOT constant redefinitions from unit tests.

Issue #697760 by sun: Replace getInfo() in tests with native phpDoc + annotations (following PHPUnit).

Issue #2246665 by jaredsmith, martin107, YesCT, filijonka, tstoeckler: Typehint with Drupal\Core\Language\LanguageInterface instead Drupal\Core\Language\Language.

Issue #2178795 by tim.plunkett, jlbellido, YesCT, Berdir, Xano: Allow DiscoveryInterface::getDefinition() to throw an exception for an invalid plugin.

Issue #2039435 by Xano, Berdir | akshay.swnt22: Convert EntityManager to extend DefaultPluginManager.

Issue #2231419 by sun, jibran, Rajendar Reddy, Jalandhar, damiankloip: Merge obsolete CachedModuleHandler into ModuleHandler.

Issue #2053153 by tim.plunkett, dawehner, damiankloip, aspilicious: Allow contrib modules to provide plugins on behalf of optional modules.

Issue #1862202 by plach, Berdir, katbailey, ParisLiakos, alexpott, chx, sun, larowlan, Gábor Hojtsy, cosmicdreams, vijaycs85, YesCT, penyaskito, andypost, Albert Volkman, joelpitett: Objectify the language system.

Issue #1754246 by webflo, swentel, penyaskito, andypost, alexpott, YesCT, dagmar: Languages should be configuration entities.

Issue #1903346 by Berdir, tim.plunkett, EclipseGc: Establish a new DefaultPluginManager to encapsulate best practices.