;;; package.lisp
#-to-host
(defpackage #:custom-hash-table
  (:use))

(defpackage #:custom-hash-table.impl
  (:use #:cl)
  (:local-nicknames
   (#:target #+to-host #:cl
             #-to-host #:custom-hash-table)))

;;; code.lisp
(in-package #:custom-hash-table.impl)
(defun target:make-hash-table (...) ...)

;;; ASDs:
;;; Either have one system and expect the user to push the feature,
;;; or have two systems, one of which pushes the feature in a perform
;;; method or something like that.