emacs: Add 'guix-package-names'.

* emacs/guix-main.scm (package-names, package-names-lists): New procedures.
* emacs/guix-base.el (guix-package-names): New function.

emacs/guix-base.el

@@ -191,6 +191,20 @@ If PATH is relative, it is considered to be relative to
   "Return a list of names of available lint checkers."
   (guix-eval-read (guix-make-guile-expression 'lint-checker-names)))
 
+(guix-memoized-defun guix-package-names ()
+  "Return a list of names of available packages."
+  (sort
+   ;; Work around <https://github.com/jaor/geiser/issues/64>:
+   ;; list of strings is parsed much slower than list of lists,
+   ;; so we use 'package-names-lists' instead of 'package-names'.
+
+   ;; (guix-eval-read (guix-make-guile-expression 'package-names))
+
+   (mapcar #'car
+           (guix-eval-read (guix-make-guile-expression
+                            'package-names-lists)))
+   #'string<))
+
 
 ;;; Buffers and auto updating.
 

emacs/guix-main.scm

@@ -942,3 +942,14 @@ GENERATIONS is a list of generation numbers."
   (map (lambda (checker)
          (symbol->string (lint-checker-name checker)))
        %checkers))
+
+(define (package-names)
+  "Return a list of names of available packages."
+  (delete-duplicates
+   (fold-packages (lambda (pkg res)
+                    (cons (package-name pkg) res))
+                  '())))
+
+;; See the comment to 'guix-package-names' function in "guix-popup.el".
+(define (package-names-lists)
+  (map list (package-names)))