PHP SDK requires composer itself

[xolf]

The readme/metrics requires Composer itself. As mentioned by the maintainers of Composer, see composer/composer#11940 (comment), this is a bad design and lead to unusable composer installation if the version within the vendor folder differs from the system-wide installation.

I'm not quite sure, what are the reasons to require composer/composer itself, but not other package are requiring composer, so one should be safe to remove.

[Seldaek]

I guess it might have been a misunderstanding that this is needed for InstalledVersions to work. But that's just always available when the composer-runtime-api requirement is fullfilled so it should be fine. Removing the require on composer/composer definitely would be good.