Rename Drupal\Core\PageCache namespace

Spin-off from #2429617-239: Make D8 2x as fast: Dynamic Page Cache: context-dependent page caching (for *all* users!) → :

- page cache was moved to a middleware provided by page_cache.module
- Drupal\Core\PageCache does not contain "page cache" code, but only the interfaces and base implementations of the concept of "cache policies"
- that concept is independant of the "page cache" functionality, and has nothing to do with page_cache.module's PageCache middleware class. It is primarily used to determine a response's Cache-Control headers (FinishResponseSubscriber::onRespond)
- then, page_cache.module and the new smart_cache.module both leverage cache policies as well to control their own scope.

Thus, Drupal\Core\PageCache should be something like ResponseCacheControl or ResponseCachePolicy ?