[5.x] Fix autoloading when addon has multiple service providers

[duncanmcclean]

This pull request changes the "base path" used when autoloading addon components, to prevent addons with multiple service providers from booting the same stuff multiple times.

Fixes #11082.

[jasonvarga]

I've pushed up some changes so that "root level items" like config, routes, views, blueprints, etc will only get booted by the root provider.

I added handling so that if you have multiple providers at the root level, the root level items only get booted once.

So, now nested providers can have autoloadable things relative to them like tags, actions, etc.

|-- src
|   |-- Tags
|       |-- TagOne.php
|   |-- ServiceProvider.php             <-- this will load TagOne, routes, config, lang, views
|   |-- AnotherServiceProvider.php      <-- this will only load stuff inside its $tags/$fieldtypes/etc properties
|   |-- Modules
|       |-- Something
|           |-- Tags
|           |   |-- AnotherTag.php
|           |-- ServiceProvider.php     <-- this will load AnotherTag, but not routes/config/lang/views
|-- routes
|   |-- web.php
|-- config
|   |-- addon.php 
|-- resources
    |-- lang
    |-- views

[edalzell]

Gave this a try and my "nested" routes aren't loading.

I have routes like so:

And the Stripe ones aren't loaded.

I haven't checked the configs/blueprints yet.