As well as ExplodedImage (which feeds into JRT file-system) there is another mechanism by which module resources are read. This is ExplodedModuleReader in ModuleReferences.

To support preview mode, the module reader must preferentially locate classes/resources in the META-INF/preview directory (if it exists).

Unlike JRT file system, this mechanism:
* Doesn't need to look like a file system (no directory structure)
* Must use file:/ URIs (unable to use jrt:/ URIs)

Exploded modules are used in multiple ways (no only for system modules) so care must be taken to restrict preview mode semantics to system modules only.