Make the org.wildfly.extension.elytron module private

The org.wildfly.extension.elytron module was originally public as there were some classes in the elytron subsystem that custom components needed access to in order to implement them.

There are currently two Elytron custom components that still rely on needing to implement interfaces from the Elytron subsystem:

• `custom-principal-transformer` relies on a `org.wildfly.extension.elytron.capabilities.PrincipalTransformer` implementation
• `custom-credential-security-factory` relies on a `org.wildfly.extension.elytron.capabilities.CredentialSecurityFactory` implementation

It's now possible to implement these custom components without needing to implement these classes so we can mark the module as private.