Remove @dojo/interfaces

[matt-gadd]

Given @dojo/interfaces is now pretty much redundant, we can get rid of it out of our downstream dependencies. To do this for non-cli packages, we simply need to add the remaining stray loader types to @dojo/core (PR here: https://github.com/dojo/core/pull/375). We should then be able to remove the dependency on @dojo/interfaces from all of the following:

• [x] dojo/core
• [x] dojo/examples
• [x] dojo/i18n
• [x] dojo/interop
• [x] dojo/routing
• [x] dojo/stores
• [x] dojo/streams
• [x] dojo/widget-core
• [x] dojo/widgets
• [x] dojo/test-extras
• [x] dojo/web-editor

After the above, the cli commands currently have a dependency on @dojo/interfaces for the cli typings, if we remove the current circular dependency issue due to bundling @dojo/cli-create-app with @dojo/cli (issue here: https://github.com/dojo/cli/issues/183), we can go ahead and just move these types back into @dojo/cli and make the packages below just rely on @dojo/cli as a dev dep for the typings.

• [ ] dojo/cli-build
• [ ] dojo/cli-create-app
• [ ] dojo/cli-create-widget
• [ ] dojo/cli-export-project
• [ ] dojo/cli-test-intern
• [ ] dojo/cli-build-app
• [ ] dojo/cli-create-theme

After the above, we can go ahead and remove/deprecate the @dojo/interfaces package and repo.

[umaar]

The pull requests are ready! When "@dojo/cli": "~0.4.2" is published then we can merge them