Previously I attempted to de-dup and cache any FileSystemResolver under
the same path. This created issues because each Resolver caches its
templates. Those cached templates only get compiled once into whatever
compiled_method_container is passed on its first render. So separate
compiled_method_containers must have different copies of a Template, and
therefore separate copies of each Resolvers.

I tried a few approaches where I would have a separate cache of
resolvers per-compiled-method-container, but I wasn't really successful
as in most of the places we performed the casting we did not know or
have access to the view context class and therefore compiled method
container (it isn't stored in the LookupContext or ViewPath) and trying
to add that relating created incompatibilities in some
not-quite-public-but-still-widely used APIs for using ActionView
directly.

This approach instead limits where we perform the de-dup and cache, to
only descendants of ActionView::ViewPaths. These can be assumed to also
inherit from ActionView::Rendering (I think ActionController::API is an
exception here, which should be addressed in a future refactor) which
means that they should all share the same compiled_method_container and
therefore can all be cached safely together.

This should continue to ensure we don't leak memory from
{append,prepend}_view_path at runtime, while maintaining compatibility
with some of the more specialized uses of ActionView::Base as a
standalone API. However this leaves open the possibility (which has
always existed) of using that standalone API in a "leaky" way.

* Invoke callback only once building view Resolvers

Previously we would run the callback after each resolver built. We only
need to possibly recreate the watcher once all new resolvers have been
instantiated.

Additionally this now only invokes the callback when a new resolver has
actually been built, if one was found in the cache no work is needed.

Co-authored-by: Rafael Mendonça França <rafael@franca.dev>