php.buildPecl: Allow PECLs to depend on other PECLs

Some PECLs depend on other PECLs and, like internal PHP extension
dependencies, need to be loaded in the correct order. This makes this
possible by adding the argument "peclDeps" to buildPecl, which adds
the extension to buildInputs and is treated the same way as
internalDeps when the extension config is generated.