Add autoload paths for app plugins.

[ADmad]

This change avoids having to update the app's composer.json to add autoload paths for app plugins under the "plugins" folder.

I have been using a custom composer plugin which does the same for quite a while for my apps.

[ADmad]

After this we can update bake's plugin creating command to run composer dump-autoload instead of modifying the app's composer.json.

[ADmad]

One can still manually update composer.json and any existing namespace under psr-4 autoload will be left unchanged (as seen for Foo\\ and Foo\Test\\ in the test case).

[markstory]

This is great :clap: We should announce this on the bakery as it is a good quality of life improvement.

[ADmad]

This is targeting 2.x. Would have to be backported to 1.x for Cake 4 apps.

[LordSimal]

If we are going to backport it to 1.x I will definitely make a separate video showcasing the differences between the old and the new system. But indeed this is an awesome feature 👍🏻

[LordSimal]

This should be merged before we release Cake5