This means php packages can now refer to other php packages by looking
them up in the php.packages attribute and gets rid of the internal
recursive set previously defined in php-packages.nix. This also means
that in applications where previously both the php package and the
corresponding version of the phpPackages package set had to be
specified, the php package will now suffice.
This also adds the phpWithExtensions parameter to the
php-packages.nix, which can be used by extensions that need a fully
featured PHP executable.
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