import: utils: Add hash-ref*.

With the change to guile-json version 3, JSON objects are represented as hash
tables, rather than alists. The cpan importer uses assoc-ref* on a hash table,
so add an equivalent function for hash tables.

* guix/import/utils.scm (hash-ref*): New procedure.

guix/import/utils.scm

@@ -47,6 +47,7 @@
 
             flatten
             assoc-ref*
+            hash-ref*
 
             url-fetch
             guix-hash-url
@@ -116,6 +117,15 @@ recursively apply the procedure to the sub-list."
       (assoc-ref alist key)
       (apply assoc-ref* (assoc-ref alist key) rest)))
 
+(define (hash-ref* hash-table key . rest)
+  "Return the value for KEY from HASH-TABLE.  For each additional key specified,
+recursively apply the procedure to the sub-hash-table."
+  (if (hash-table? hash-table)
+      (if (null? rest)
+          (hash-ref hash-table key)
+          (apply hash-ref* (hash-ref hash-table key) rest))
+      #f))                              ; For consistency with assoc-ref*
+
 (define (url-fetch url file-name)
   "Save the contents of URL to FILE-NAME.  Return #f on failure."
   (parameterize ((current-output-port (current-error-port)))