Add extension paths before initialize_cache

solidus_support v0.9.0, released on May 26, 2022, included a commit that fixes
the load order of adding extension paths. See
202e77c.
The commit depends on the `initialize_dependency_mechanism` Rails initializer
(https://github.com/rails/rails/blob/127dd06df66552dd272eea7832f8bb205cf6fd01/railties/lib/rails/application/bootstrap.rb#L68).
However, that initiliazer was removed in Rails 7.0.3
(rails/rails@0375657).

The extension paths initializer has been updated to run before
`initialize_cache`. `initialize_cache` runs 1) after the Solidus engines have
already loaded BUT 2) before Rails has added the paths to `$LOAD_PATH`.
Normally, it would be sufficient to run the initializer before the
`set_load_path` initializer. However, external gems such as Deface may also
change the load paths before `set_load_path`. To ensure that our
extension paths are not affected by those gems, we work around those gems by
adding our paths before `initialize_cache`, which is the Rails initializer
called before `set_load_path`.

References
----------

* `bin/rails initializers` - Shows list of active initializers
* List of Rails initializers: https://guides.rubyonrails.org/configuring.html#initializers
* Engine paths configuration - See Paths section in https://api.rubyonrails.org/v7.0.3/classes/Rails/Engine.html.

lib/solidus_support/engine_extensions.rb

@@ -82,7 +82,24 @@ def solidus_subscribers_root
       #
       # @see #load_solidus_decorators_from
       def enable_solidus_engine_support(engine)
-        initializer "#{name}_#{engine}_paths", before: :initialize_dependency_mechanism do
+        # In the past, view and controller paths of Solidus extensions were
+        # added at extension loading time. As a result, if an extension
+        # customizing a Solidus engine is loaded before that engine
+        # (for example, the extension is declared before the `solidus_frontend`
+        # engine in the `Gemfile`), then the extension's paths for that Solidus
+        # engine wouldn't be loaded.
+
+        # The initializer below runs before `initialize_cache` because
+        # `initialize_cache` runs 1) after the Solidus engines have already
+        # loaded BUT 2) before Rails has added the paths to `$LOAD_PATH`.
+        # Normally, it would be sufficient to run the initializer below before
+        # the `set_load_path` initializer. However, external gems such as
+        # Deface may also change the load paths immediately before
+        # `set_load_path`. To ensure that our extension paths are not affected
+        # by those gems, we work around those gems by adding our paths before
+        # `initialize_cache`, which is the Rails initializer called before
+        # `set_load_path`.
+        initializer "#{name}_#{engine}_paths", before: :initialize_cache do
           if SolidusSupport.send(:"#{engine}_available?")
             paths['app/controllers'] << "lib/controllers/#{engine}"
             paths['app/views'] << "lib/views/#{engine}"