Automatically detect PHP extension directory

When adding, for instance, the XDebug module, one currently has to write:

```puppet
php::module::ini { 'pecl-xdebug':
  zend => '/usr/lib64/php/modules',
}
```

With this PR it becomes easier to include Zend extensions like XDebug:

```puppet
php::module::ini { 'pecl-xdebug':
  zend => true,
}
```

In this case, the PHP extension directory is automatically detected through a Puppet fact. If you need to override the module directory, you can still do so by supplying a string for the `zend` property:

```puppet
php::module::ini { 'pecl-xdebug':
  zend => '/some/other/php/modules/dir',
}
```