Removes PIXI.loaders namespace, adds consistent loader plugin structure (v5) (#4471) ...

* Remove inline loader middleware install

* Removes PIXI.loaders namespace, adds PIXI.Loader.registerPlugin

Next Restructure for v5 (#4387) ...

* Reorganized files

* Adds new files

* Adds new files

* Updated to next-lerna

* Linting fixes, disable deprecations

* Created bundles and plugins folders

* Adds polyfill plugin

* Test pass

* Better build dest selection

* Externalized fragment shaders and vertex shaders

* Renamed plugins to packages

* Creates core package, updates Rollup version

* Reorganized pixi.js into packages

* Added packages, readmes, licenses for all packages

* Resolve minor build, dependency issues

* Ignore non-first comments with uglify

* Fixes fragment dependencies

* Adds settings, constants, utils, math, display, app

* Fixes import issues with core

* Fixes build all modules, updates deprecations

* Fixes listing issues

* Temporarily disable coverage

* Broke up tests

* Rename GeometryBuffer to Buffer

* Fixes tests for all

* Remove placeholder test

* Adds test for constants

* Add test for accessibility

* Created integrations tests

* Travis-friendly test concurrency

* Un-did concurrency for tests

* Better tester shebang

* Temporarily disable tests for mesh

* Log out floss error

* Moves filters to new filters root folder

* Combines premultiply, deprecates PIXI.extras namespace

* Documentation fixes

* Moves integration tests to tools

* TimeLimiter test is too unstable for slow environments