Fix expansion of layout variables inside suffix specifications

The `transient-define-groups' macro saves the parsed suffixes in the
`transient--layout' property, but later when `transient--parse-child'
is used by `transient-define-prefix', that expects the unparsed group
or suffix specifications in the symbol's value cell.  The latter is an
older approach, which was supposed to be replaced when the macro was
introduced.  Because that didn't happen, the macro never worked.

Additionally change `transient--parse-child' to not deal with embedded
symbols, except for quoting them.  It is now `transient--init-child's
responsibility to fill in the indirectly specified suffixes.  In other
words it happens when a prefix is invoked, not when it is defined.
That has the advantage that indirectly defined suffixes can be
modified the same way as directly specified.

Modifying indirectly specified suffixes) now affects all prefixes
that use those suffixes, without having to re-evaluate the prefix
definitions.

lisp/transient.el

@@ -881,6 +881,18 @@ to the setup function:
             (list ,@(cl-mapcan (lambda (s) (transient--parse-child name s))
                                suffixes))))))
 
+(defmacro transient-define-groups (name &rest groups)
+  "Define one or more GROUPS and store them in symbol NAME.
+GROUPS, defined using this macro, can be used inside the
+definition of transient prefix commands, by using the symbol
+NAME where a group vector is expected.  GROUPS has the same
+form as for `transient-define-prefix'."
+  (declare (debug (&define name [&rest vectorp]))
+           (indent defun))
+  `(put ',name 'transient--layout
+        (list ,@(cl-mapcan (lambda (group) (transient--parse-child name group))
+                           groups))))
+
 (defmacro transient-define-suffix (name arglist &rest args)
   "Define NAME as a transient suffix command.
 
@@ -1009,12 +1021,7 @@ example, sets a variable, use `transient-define-infix' instead.
 
 (defun transient--parse-child (prefix spec)
   (cl-etypecase spec
-    (symbol  (let ((value (symbol-value spec)))
-               (if (and (listp value)
-                        (or (listp (car value))
-                            (vectorp (car value))))
-                   (cl-mapcan (lambda (s) (transient--parse-child prefix s)) value)
-                 (transient--parse-child prefix value))))
+    (symbol  (list (list 'quote spec)))
     (vector  (and-let* ((c (transient--parse-group  prefix spec))) (list c)))
     (list    (and-let* ((c (transient--parse-suffix prefix spec))) (list c)))
     (string  (list spec))))
@@ -1888,6 +1895,8 @@ value.  Otherwise return CHILDREN as is."
 
 (defun transient--init-child (levels spec)
   (cl-etypecase spec
+    (symbol  (cl-mapcan (lambda (c) (transient--init-child levels c))
+                        (get spec 'transient--layout)))
     (vector  (transient--init-group  levels spec))
     (list    (transient--init-suffix levels spec))
     (string  (list spec))))