Take input files into account when determining last-modified revision

Some files are not distributed with the package but still affect the
files that are.  An texi manual for example may include another texi
file that is not a manual in its own right, but if the included file
is modified, that still effects the generated info file.

package-build.el

@@ -1320,6 +1320,13 @@ order and can have the following form:
   returned alist.  Files matched by later elements are not
   affected.
 
+- (:inputs GLOB...)
+
+  A list that begins with `:inputs' specifies files, which are not
+  to be included in the package, but when modified still trigger a
+  new package version.  I.e., this function ignores this element,
+  but `package-build--spec-globs' does not.
+
 - (:rename SRC DEST)
 
   A list that begins with `:rename' causes the file SRC to be
@@ -1350,6 +1357,7 @@ SPEC is a full files spec as stored in a recipe object."
   (let (include exclude)
     (dolist (entry spec)
       (pcase (car-safe entry)
+        (:inputs)
         (:exclude
          (dolist (entry (cdr entry))
            (push entry exclude)))
@@ -1422,6 +1430,9 @@ FILES is a list of (SOURCE . DEST) relative filepath pairs."
                 ((and `(:exclude . ,globs)
                       (guard (cl-every #'stringp globs)))
                  (mapcan (lambda (g) (toargs g t)) globs))
+                ((and `(:inputs . ,globs)
+                      (guard (cl-every #'stringp globs)))
+                 (mapcan #'toargs globs))
                 ((and `(,dir . ,globs)
                       (guard (stringp dir))
                       (guard (cl-every #'stringp globs)))

package-recipe.el

@@ -201,13 +201,14 @@ file is invalid, then raise an error."
         (when (eq (car spec) :defaults)
           (setq spec (cdr spec)))
         ;; All other elements have to be strings or lists of strings.
-        ;; A list whose first element is `:exclude' is also valid.
+        ;; Lists whose first element is `:exclude' or `:inputs' are
+        ;; also valid.
         (dolist (entry spec)
           (unless (cond ((stringp entry)
                          (not (equal entry "*")))
                         ((listp entry)
                          (and-let* ((globs (cdr entry)))
-                           (and (or (eq (car entry) :exclude)
+                           (and (or (memq (car entry) '(:exclude :inputs))
                                     (stringp (car entry)))
                                 (seq-every-p (lambda (glob)
                                                (and (stringp glob)