WebEngine devmode is broken for modules that do not declare a module.xml file

When Dev mode is activated, Nuxeo WebEngine uses a separated classloader to reload the modules.
Because the main root map uses Classes as key is does only work when the fallback on module.xml is available (using String instead of a class).

=> move the map to <String,ModuleConfiguration> so that WebEngine module that do not use a module.xml can still start when dev mode is activate.