gnu: emacs: Update to 27.1.

* gnu/packages/emacs.scm (emacs): Update to 27.1.
[source]: In snippet, use 'find-files' to find 'eshell/esh-groups.el'
if it exists, but gracefully handle its absence.
[arguments]: Add "--with-cairo" to configure-flags.  Add 'restore-emacs-pdmp'
and 'make-compressed-files-writable' phases.  Explicitly return #t from
'install-site-start' phase.
[inputs]: Add cairo, pango, harfbuzz, jansson, and gmp.  Remove imagemagick.
(emacs-next): Remove variable.
(emacs-minimal, emacs-xwidgets): Use 'package/inherit'.
[arguments]: Remove 'restore-emacs-pdmp' phase.
(emacs-no-x): Use 'package/inherit'.
[arguments]: Remove "--with-cairo" from configure-flags.  Remove
'restore-emacs-pdmp' phase.
[inputs]: Remove cairo, pango, and harfbuzz.
(emacs-no-x-toolkit): Use 'package/inherit'.
[arguments]: Remove 'restore-emacs-pdmp' phase.  Remove superfluous
quasiquote+unquote-splicing pair.
(emacs-wide-int): Use 'package/inherit'.
(guile-emacs): Use 'package/inherit'.
[arguments]: Remove "--with-cairo" from configure-flags.  Remove
'restore-emacs-pdmp' phase.
* gnu/packages/patches/emacs-ignore-empty-xim-styles.patch,
gnu/packages/patches/emacs-source-date-epoch.patch: Adapt to
apply cleanly to emacs-27.1.
* gnu/packages/patches/emacs-exec-path.patch: Replace with the previous
contents of 'emacs27-exec-path.patch', but remove the superfluous comment
about it being "like emacs-exec-path.patch, but for Emacs 27".
* gnu/packages/patches/emacs27-exec-path.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Remove 'emacs27-exec-path.patch'.

gnu/packages/patches/emacs-exec-path.patch

@@ -4,15 +4,13 @@ with things like GCC being referenced.
 
 --- a/lisp/loadup.el
 +++ b/lisp/loadup.el
-@@ -64,6 +64,11 @@
- 			    (expand-file-name "textmodes" dir)
- 			    (expand-file-name "vc" dir)))))
- 
-+;; Reset 'exec-path' so we don't capture the build-time $PATH in the
-+;; 'emacs' executable.
-+(setq-default exec-path nil)
-+(setq exec-path nil)
-+
- (if (eq t purify-flag)
-     ;; Hash consing saved around 11% of pure space in my tests.
-     (setq purify-flag (make-hash-table :test 'equal :size 70000)))
+@@ -481,7 +481,8 @@ lost after dumping")))
+                         ((equal dump-mode "dump") "emacs")
+                         ((equal dump-mode "bootstrap") "emacs")
+                         ((equal dump-mode "pbootstrap") "bootstrap-emacs.pdmp")
+-                        (t (error "unrecognized dump mode %s" dump-mode)))))
++                        (t (error "unrecognized dump mode %s" dump-mode))))
++          (exec-path nil))
+       (message "Dumping under the name %s" output)
+       (condition-case ()
+           (delete-file output)