Prevent :exordium-force-elpa from clobbering already installed package

When a package has already been installed (as indicated by presence in
`package-alist`), there should be no attempt to install it again. While it is
technically possible to have multiple versions of the same package installed
side by side this deviates from the existing workflow where only one version of
the package has been installed at any given time (at least when following
standard `package-upgrade`).

modules/init-force-elpa.el

@@ -48,20 +48,25 @@
      (package-read-all-archive-contents)
      (when-let* (((package-built-in-p package))
                  (builtin-version (alist-get package package--builtin-versions))
+                 (find-archive-desc
+                  (lambda ()
+                    (cl-find-if (lambda (desc)
+                                  (equal (package-desc-archive desc)
+                                         ,archive))
+                                (alist-get package
+                                           package-archive-contents))))
                  (pkg-desc (or
-                            (cl-find-if (lambda (desc)
-                                          (equal (package-desc-archive desc)
-                                                 ,archive))
-                                        (alist-get package
-                                                   package-archive-contents))
+                            (funcall find-archive-desc)
                             (progn
                               (package-refresh-contents)
-                              (cl-find-if (lambda (desc)
-                                            (equal (package-desc-archive desc)
-                                                   ,archive))
-                                          (alist-get package
-                                                     package-archive-contents)))))
+                              (funcall find-archive-desc))))
                  (archive-version (package-desc-version pkg-desc))
+                 ;; package has been previously installed, either not a
+                 ;; built-in package or a built-in package has been shadowed by
+                 ;; installation
+                 ((not (assq package package-alist)))
+                 ;; package with archive-version that is higher than the
+                 ;; built-in one
                  ((not (package-installed-p package archive-version)))
                  ((version-list-< builtin-version archive-version)))
        (package--save-selected-packages (cons package