[4.0] Improve Autoloader with composer capabilities (e.g. Class Map)

[krileon]

Joomla is replacing composer autoloader, but does not provide functionality to add a class map. So if my extension is using composer and I want to be able to reach my dependencies in my vendor folder then Joomla won't be able to unless I register my composer autoloader resulting in two autoloaders for no real reason.

Ideally we should just be able to define a path to a classmap in the XML same as we do with namespace. So currently we have the below.

<namespace path="src">Joomla\Component\Content</namespace>

This could be adjusted to support another top level node like the following.

<classmap>src/vendor/classmap.php</classmap>

Maybe a better structure can be proposed or just an attribute of namespace even like the following.

<namespace path="src" classmap="src/vendor/classmap.php">Joomla\Component\Content</namespace>

I'm also unsure if the above works in library extensions, but I would hope it would as needing a system plugin to load libraries shouldn't be necessary.

[krileon]

Any thoughts? Is there no way to load in another composers classmap from an extensions vendor dependencies into Joomlas autoloader? Am I missing something with the namespace XML element that already allows this?