Rename PHP implementation to Skillshare library

[rvanlaak]

When looking at the code related to the /implementation/php, it seems that this isn't testing PHP, but that according to the composer.json the skillshare/apollo-federation-php library is being tested. This leads to unclarity.

In addition, I've assessed all of the PHP libraries on https://graphql.org/code/#php, and it seems that the majority of them is an extension to the webonyx/graphql-php library. Maybe we can add a test suite which solely tests the webonyx library?

TODO

• add test coverage on webonyx/graphql-php implementation
• rename the /implementation/php to /implementation/php-skillshare/apollo-federation-php