[php] Use PHP composer to clone git repo

If you want to use a composer.json from GitHub you would look at this example (under the VCS section).

The package section is for packages that do not have the composer.json. However, you didn't follow that example as well or it would also have worked. Do read what it says about package repositories:

Basically, you define the same information that is included in the composer repository's packages.json, but only for a single package. Again, the minimum required fields are name, version, and either of dist or source.