Move controller classes to Controller namespaces

Problem/Motivation

Since #3311365: Use PHP attributes for route discovery has introduced route definitions via PHP attributes, this means route discovery iterates through and reflects all files/classes in the Controller namespace/directory of modules.

There are some controllers that are not in a Controller namespace / directory so they can't use Route attributes.

Steps to reproduce

Runtime:

grep -rn '_controller:' core --include="*.routing.yml" \
  | grep -v '\\Drupal\\[a-zA-Z_]*\\Controller\\' \
  | grep -v tests

Tests:

grep -rn '_controller:' core --include="*.routing.yml" \
  | grep -v '\\Drupal\\[a-zA-Z_]*\\Controller\\' \
  | grep tests

Results attached

Proposed resolution

Move some or all of these to a Controller subdirectory.

Co-ordinate with #3490484: [meta] Lots of non-plugin PHP classes in plugin directories and #3584795: Remove non-controller classes from module Controller namespaces/folders to avoid conflicts.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet