Add command to start Emacs with specific packages

* lisp/emacs-lisp/package.el (package--dependencies): Extend function
to handle and return package descriptors.
(package-isolate): Add new command.
* etc/NEWS: Announce new command.

etc/NEWS

@@ -514,6 +514,15 @@ project, that you can quickly select using 'project-switch-project'
 When non-nil, package specifications with side-effects for building
 software will be used when building a package.
 
+---
+*** New command to start an inferior Emacs loading only specific packages.
+The new command 'package-isolate' will start a new Emacs process, as
+a sub-process of Emacs where you invoke the command, in a way that
+causes the new process to load only some of the installed packages.
+The command prompts for the packages to activate in this
+sub-process, and is intended for testing Emacs and/or the packages
+in a clean environment.
+
 ** Flymake
 
 +++

lisp/emacs-lisp/package.el

@@ -2330,12 +2330,25 @@ from ELPA by either using `\\[package-upgrade]' or
       (mapc #'package-upgrade upgradeable))))
 
 (defun package--dependencies (pkg)
-  "Return a list of all dependencies PKG has.
-This is done recursively."
-  ;; Can we have circular dependencies?  Assume "nope".
-  (when-let* ((desc (cadr (assq pkg package-archive-contents)))
-              (deps (mapcar #'car (package-desc-reqs desc))))
-    (delete-dups (apply #'nconc deps (mapcar #'package--dependencies deps)))))
+  "Return a list of all transitive dependencies of PKG.
+If PKG is a package descriptor, the return value is a list of
+package descriptors.  If PKG is a symbol designating a package,
+the return value is a list of symbols designating packages."
+  (when-let* ((desc (if (package-desc-p pkg) pkg
+                      (cadr (assq pkg package-archive-contents)))))
+    ;; Can we have circular dependencies?  Assume "nope".
+    (let ((all (named-let more ((pkg-desc desc))
+                 (let (deps)
+                   (dolist (req (package-desc-reqs pkg-desc))
+                     (setq deps (nconc
+                                 (catch 'found
+                                   (dolist (p (apply #'append (mapcar #'cdr (package--alist))))
+                                     (when (and (string= (car req) (package-desc-name p))
+                                                (version-list-<= (cadr req) (package-desc-version p)))
+                                       (throw 'found (more p)))))
+                                 deps)))
+                   (delete-dups (cons pkg-desc deps))))))
+      (remq pkg (mapcar (if (package-desc-p pkg) #'identity #'package-desc-name) all)))))
 
 (defun package-strip-rcs-id (str)
   "Strip RCS version ID from the version string STR.
@@ -2625,6 +2638,57 @@ will be deleted."
                   removable))
         (message "Nothing to autoremove")))))
 
+(defun package-isolate (packages &optional temp-init)
+  "Start an uncustomised Emacs and only load a set of PACKAGES.
+If TEMP-INIT is non-nil, or when invoked with a prefix argument,
+the Emacs user directory is set to a temporary directory."
+  (interactive
+   (cl-loop for p in (cl-loop for p in (package--alist) append (cdr p))
+	    unless (package-built-in-p p)
+	    collect (cons (package-desc-full-name p) p) into table
+	    finally return
+	    (list (cl-loop for c in (completing-read-multiple
+                                     "Isolate packages: " table
+                                     nil t)
+		           collect (alist-get c table nil nil #'string=))
+                  current-prefix-arg)))
+  (let* ((name (concat "package-isolate-"
+                       (mapconcat #'package-desc-full-name packages ",")))
+         (all-packages (delete-consecutive-dups
+                        (sort (append packages (mapcan #'package--dependencies packages))
+                              (lambda (p0 p1)
+                                (string< (package-desc-name p0) (package-desc-name p1))))))
+         initial-scratch-message package-load-list)
+    (with-temp-buffer
+      (insert ";; This is an isolated testing environment, with these packages enabled:\n\n")
+      (dolist (package all-packages)
+        (push (list (package-desc-name package)
+                    (package-version-join (package-desc-version package)))
+              package-load-list)
+        (insert ";; - " (package-desc-full-name package))
+        (unless (memq package packages)
+          (insert " (dependency)"))
+        (insert "\n"))
+      (insert "\n")
+      (setq initial-scratch-message (buffer-string)))
+    (apply #'start-process (concat "*" name "*") nil
+           (list (expand-file-name invocation-name invocation-directory)
+                 "--quick" "--debug-init"
+                 "--init-directory" (if temp-init
+                                        (make-temp-file name t)
+                                      user-emacs-directory)
+                 (format "--eval=%S"
+                         `(progn
+                            (setq initial-scratch-message ,initial-scratch-message)
+
+                            (require 'package)
+                            ,@(mapcar
+                               (lambda (dir)
+                                 `(add-to-list 'package-directory-list ,dir))
+                               (cons package-user-dir package-directory-list))
+                            (setq package-load-list ',package-load-list)
+                            (package-initialize)))))))
+
 
 ;;;; Package description buffer.