Move interface out of plugin folders

Updated: Comment #1

Problem/Motivation

From #2151829-13: Doctrine annotation parsing takes an unacceptable amount of time/memory on install → . There's no need to parse an interface for annotations.

Here's a list of offending interfaces (there might be more -- I searched for Entity/**/*Interface.php and Plugin/*/**/*Interface.php -- are there any more plugin namespaces in core?):

modules/entity_reference/lib/Drupal/entity_reference/Plugin/Type/Selection/SelectionInterface.php 
modules/views/lib/Drupal/views/Plugin/views/display/DisplayRouterInterface.php                     modules/views/lib/Drupal/views/Plugin/views/wizard/WizardInterface.php
modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php

Proposed resolution

We should move interfaces outside plugin folders so that the Doctrine annotation parser doesn't hit them. This is way easier than teaching Doctrine to skip them.

Remaining tasks

N/A

User interface changes

N/A

API changes

Minor, relatively unused interfaces are moving.