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 #2600926 by benjy, mikeryan, phenaproxima, hussainweb, alvar0hurtad0: Allow annotations to inherit across namespaces

(cherry picked from commit 09c51315)

Issue #2337191 by tim.plunkett, dawehner, jibran, yched, Wim Leers, catch: Split up EntityManager into many services

Issue #2454649 by Aki Tendo, dawehner: Cache Optimization and hardening -- [PP-1] Use assert() instead of exceptions in Cache::merge(Tags|Contexts)

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

Issue #2422815 by Xano, fgm, tim.plunkett, dawehner: Don't initialize the discovery object in plugin managers, unless needed

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

Issue #2414991 by alexpott: Prevent hook_config_schema_info_alter from adding or removing definitions

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 #2234439 by jhodgdon, cosmicdreams, xjm: Add a section for the plugin system to the API doc landing page.

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

Issue #2169447 by damiankloip, sun, longwave: DX: Supply CacheBackendInterface::CACHE_PERMANENT as Cache::PERMANENT.

Issue #2170435 by Xano, EclipseGc: PluginManagerBase::processDefinition() fails when plugin definitions are not arrays.

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 #2028511 by damiankloip, dawehner: Allow plugin derivative classes to use services from the container.

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.