Fix the load order issues of Sweeping module

### WHAT

Remove `Concern` from the `Sweeping` module so we can directly extend it in `ActionController::Base`

### WHY

Basically this is what happens, in the context of `ActionController` when the app is booting:
1) `Base` loads and includes the `Caching` module
2) `rails-observers` railtie is then triggered to include `Caching::Sweeping` concern as a dependency of `Caching`, but it doesn't actually get included because `Caching` is a concern too.
3) However `Caching` was already included in step 1, so `Caching::Sweeping` never gets included in `Base` and people run into this error:

https://github.com/rails/rails-observers/issues/4

Note: the tests for this were working before because it didn't use the railtie to load `Sweeping`. It just directly loaded the files with `Sweeping` in them before it loaded `ActionController::Base`.