Hide is_imported from shellfu()

This was never intended to be public (and never documented).

src/shellfu.sh.skel

             # Import module named $2
             #
             local mname=$2
-            shellfu is_imported "$mname" && return 0
+            shellfu _is_imported "$mname" && return 0
             if shellfu _do_import "$mname";
             then
                 SHELLFU_IMPORTED="$SHELLFU_IMPORTED${SHELLFU_IMPORTED:+:}$mname"
             local mname=$2
             ( shellfu _do_import "$mname" )
             ;;
-        is_imported)
-            #
-            # True if module $2 is already imported
-            #
-            local mname=$2
-            echo "$SHELLFU_IMPORTED" | tr : \\n | grep -qx "$mname"
-            ;;
 
         _do_import)
             #
             fi
             return $es
             ;;
+        _is_imported)
+            #
+            # True if module $2 is already imported
+            #
+            local mname=$2
+            echo "$SHELLFU_IMPORTED" | tr : \\n | grep -qx "$mname"
+            ;;
 
         _list_mfiles)
             #