Add an additional non-standard location to the metadata pool where metadata will be read from.
If directory contains a "xml", "xmls", "yaml" or "icons" subdirectory (or all of them), those paths will be added to the
search paths instead.
| this |
An instance of Pool. |
| directory |
An existing filesystem location. |
| format_style |
The expected format style of the metadata, e.g. as_format_style_collection |