Installing the ClassLoader component with Composer makes installing the ClassLoader component largely irrelevant

[hpierce1102]

Installing the ClassLoader component by using Composer brings in Composer's own ClassLoader, which has significant overlap with Symfony's ClassLoader.

There's a note on The PSR-4 Class Loader page suggesting that the loader should only be used when Composer is not available - but it looks like this is relevant to the PSR-0 and ClassMap loaders too. I think it would be more clear if this note were moved to the introduction page.

It would also make things more clear if we tweaked the install instructions to promote $ git clone rather than using $ composer require symfony/class-loader. This would be more consistent with the documentation pages that follow that use require_once '/path/to/src/Symfony/Component/ClassLoader/ClassLoader.php'; instead of require_once vendor/autoload.php

[javiereguiluz]

I agree ... but given that this component will be deprecated soon, in #7383 I propose to add just a note about this, instead of making all the changes proposed in this PR.