Unfortunately, the way dropAll works is that it makes a database snapshot to find all the objects liquibase knows about and then drops them. Dblinks, package, functions, collections, etc get beyond the scope of what liquibase tries to handle and so it doesn't know to drop them.
I'm not sure if there is an easy SQL command to drop everything in oracle. You could look at adding support in the liquibase-oracle extension for snapshotting and dropping the object types you are interested in, but that will take a bit of java coding.
Nathan