Allow other plugins to provide plugin data too

The filter should return an array, this could be an empty array or an array containing as a minimum the below for each plugin, obviously with data for the provides/depends key where applicable:
```php
array(
	'plugin-dir/file.php' => array(
		'Name' => 'Plugin name',
		'Provides' => '',
		'Depends' => '',
	)
)
```
@jrfnl I think it would make more sense to change this up so that the `$all_plugins` array is filtered:

```php
$all_plugins = array_merge( get_plugins(), get_mu_plugins() );
$all_plugins = apply_filters( 'plugin_dependencies_all_plugins', $all_plugins );
```