I can't find any obvious error in the import definition of package.lisp.
Perhaps it's due to the internal hash table returning NIL for CL:NIL and a
second return value not being checked.
* (defpackage "TEST" (:use)) (in-package "TEST")
* (cl:import 'cl:nil)
debugger invoked on a COMMON-LISP:UNBOUND-VARIABLE in thread 2329:
The variable NIL is unbound.