Don't run gpg when loading package.el

* lisp/emacs-lisp/package.el (package-check-signature): Don't run
gpg on startup, but just default to `allow-unsigned'.
(package-check-signature): New function to check whether a OpenPGP
configuration is found when `allow-unsigned'.
(package--check-signature-content, package--check-signature)
(package--download-one-archive, package-refresh-contents)
(package-install-from-archive): Use function instead of variable
throughout.
* doc/emacs/package.texi (Package Installation): Document this.

lisp/emacs-lisp/package.el

@@ -331,15 +331,13 @@ default directory."
   :risky t
   :version "26.1")
 
-(defcustom package-check-signature
-  (if (and (require 'epg-config)
-           (epg-find-configuration 'OpenPGP))
-      'allow-unsigned)
+(defcustom package-check-signature 'allow-unsigned
   "Non-nil means to check package signatures when installing.
 More specifically the value can be:
 - nil: package signatures are ignored.
-- `allow-unsigned': install a package even if it is unsigned,
-  but if it is signed and we have the key for it, verify the signature.
+- `allow-unsigned': install a package even if it is unsigned, but
+  if it is signed, we have the key for it, and OpenGPG is
+  installed, verify the signature.
 - t: accept a package only if it comes with at least one verified signature.
 - `all': same as t, except when the package has several signatures,
   in which case we verify all the signatures.
@@ -353,6 +351,18 @@ contents of the archive."
   :risky t
   :version "27.1")
 
+(defun package-check-signature ()
+  "Check whether we have a usable OpenPGP configuration.
+If true, and `package-check-signature' is `allow-unsigned',
+return `allow-unsigned', otherwise return the value of
+`package-check-signature'."
+  (if (eq package-check-signature 'allow-unsigned)
+      (progn
+        (require 'epg-config)
+        (and (epg-find-configuration 'OpenPGP)
+             'allow-unsigned))
+    package-check-signature))
+
 (defcustom package-unsigned-archives nil
   "List of archives where we do not check for package signatures."
   :type '(repeat (string :tag "Archive name"))
@@ -1279,15 +1289,15 @@ errors."
       (dolist (sig (epg-context-result-for context 'verify))
         (if (eq (epg-signature-status sig) 'good)
             (push sig good-signatures)
-          ;; If package-check-signature is allow-unsigned, don't
+          ;; If `package-check-signature' is allow-unsigned, don't
           ;; signal error when we can't verify signature because of
           ;; missing public key.  Other errors are still treated as
           ;; fatal (bug#17625).
-          (unless (and (eq package-check-signature 'allow-unsigned)
+          (unless (and (eq (package-check-signature) 'allow-unsigned)
                        (eq (epg-signature-status sig) 'no-pubkey))
             (setq had-fatal-error t))))
       (when (or (null good-signatures)
-                (and (eq package-check-signature 'all)
+                (and (eq (package-check-signature) 'all)
                      had-fatal-error))
         (package--display-verify-error context sig-file)
         (signal 'bad-signature (list sig-file)))
@@ -1318,7 +1328,7 @@ else, even if an error is signaled."
       :async async :noerror t
       ;; Connection error is assumed to mean "no sig-file".
       :error-form (let ((allow-unsigned
-                         (eq package-check-signature 'allow-unsigned)))
+                         (eq (package-check-signature) 'allow-unsigned)))
                     (when (and callback allow-unsigned)
                       (funcall callback nil))
                     (when unwind (funcall unwind))
@@ -1602,7 +1612,7 @@ similar to an entry in `package-alist'.  Save the cached copy to
            (local-file (expand-file-name file dir)))
       (when (listp (read content))
         (make-directory dir t)
-        (if (or (not package-check-signature)
+        (if (or (not (package-check-signature))
                 (member name package-unsigned-archives))
             ;; If we don't care about the signature, save the file and
             ;; we're done.
@@ -1654,7 +1664,7 @@ downloads in the background."
   (let ((default-keyring (expand-file-name "package-keyring.gpg"
                                            data-directory))
         (inhibit-message (or inhibit-message async)))
-    (when (and package-check-signature (file-exists-p default-keyring))
+    (when (and (package-check-signature) (file-exists-p default-keyring))
       (condition-case-unless-debug error
           (package-import-keyring default-keyring)
         (error (message "Cannot import default keyring: %S" (cdr error))))))
@@ -1901,7 +1911,7 @@ if all the in-between dependencies are also in PACKAGE-LIST."
          (file (concat (package-desc-full-name pkg-desc)
                        (package-desc-suffix pkg-desc))))
     (package--with-response-buffer location :file file
-      (if (or (not package-check-signature)
+      (if (or (not (package-check-signature))
               (member (package-desc-archive pkg-desc)
                       package-unsigned-archives))
           ;; If we don't care about the signature, unpack and we're