With PSR-0, the file path is an exact reflection of the class namespace, with the exception of underscores also being replaced with directory separators, in the designated directory. There is no prefix replacement. This is counter to how the JLoader::registerPrefix() in the CMS works, where the prefix is stripped off, then loaded from the directory by splitting on CamelCase. So JFooBar (registered with a root of libraries/joomla/) can be found at libraries/joomla/foo/bar.php
Then comes PSR-4, which is actually closer to how the Joomla prefix-based loader works. The registered prefix gets stripped from the class name, and the remaining part is then transformed into a class path.
The files in the form package need to be moved to a src/ directory, then have the class names (and file names) changed to (for example) Jooml\Form\Field\ListField, etc. This will allow us to use a PSR-4 autoloader and then everything should work fine. This change would require a major version increment. Also, all v2 packages of the framework are licensed as LGPL 2.1+ (as a result of OSM's recent decision) so this is a good time to make the other breaking changes (such as the class name changes) and then release v2.