Convert `MetadataHandler` to an abstract base class and add an
`is_enabled` class method that allows every metadata handler to
independently report whether it is enabled based on its configuration.
This avoids the need for global variables in the config module, allowing
us to change the enabled state of a metadata handler at runtime if
needed.
Guarantee that cache is initialized during startup, and only once,
instead of every time a `MetadataHandler` object is instantiated.
Also, improve logic to determine `fixtures` paths.
