core: Fix class loader visiting

The mechanism that manages loadable directories and authenticators uses
a set to remember which class loaders have already been used. This
mechanism should add visited class loaders after using them, but it
doesn't. This patch fixes that.

Change-Id: I70a9905cf0516111962b3e573cce03ee4e53f589
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>