Custom SPI module failing with java.lang.NoClassDefFoundError message after migration from Keycloak to RH-SSO

Issue

During migration from Keycloak to RH-SSO, Custom SPI module used with Keycloak community version edition is failing when used in RH-SSO environment, and preventing RH-SSO instance from starting with java.lang.NoClassDefFoundError error message:

2020-11-11 22:01:04,288 WARN  [org.jboss.modules.define] (ServerService Thread Pool -- 60) Failed to define class org.example.authenticators.ExampleUsernamePasswordFormFactory in Module "deployment.example.jar" from Service Module Loader: java.lang.NoClassDefFoundError: Failed to link org/Example/authenticators/ExampleUsernamePasswordFormFactory (Module "deployment.example.jar" from Service Module Loader): org/keycloak/authentication/authenticators/browser/UsernamePasswordFormFactory