bug#53885: Autoload package-installed-p

[Corwin Brust]

Commentary:

If the change is accpted, please consider fixing this for emacs-28.
The reproducer represents a common use-case, e.g. see bug #31397.
Moverover, it is a one-line change unlikely to affect anything aside
this and similar use-cases.

I will submit a patch as soon as I have a BugID from the tracker.
Although this change is rather trivial I need to practice this
process.

Reproducer:

NOTE: Make sure you have a folder beneth your home directory called
"elpa" and that it is empty.

Launch emacs -Q
Insert and evaluate:

(progn
  (setq package-user-dir "~/elpa")
  (setq package-archives
'(("MELPA" .
   "https://melpa.org/packages/";)))
  (require 'package)
  (unless (package-installed-p 'use-package)
    (package-install 'use-package)))

Expected result:

 Packages are installed under ~/elpa

Actual Result:

Debugger entered--Lisp error: (void-function package-installed-p)
  (package-installed-p 'use-package)
  (if (package-installed-p 'use-package) nil (package-install 'use-package))
  (progn (if (package-installed-p 'use-package) nil (package-install
'use-package)))
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  command-execute(eval-last-sexp)

Work-around:

(progn
  (setq package-user-dir "~/elpa")
  (setq package-archives
'(("MELPA" .
   "https://melpa.org/packages/";)))
  (require 'package) ;; workaround
  (unless (package-installed-p 'use-package)
    (package-install 'use-package)))

--

In GNU Emacs 28.0.91 (build 6, x86_64-w64-mingw32)
 of 2022-02-05 built on AVALON
Repository revision: 821c240075e3b850a61207d18d520126292421ce
Repository branch: emacs-28-nt-dist-build
Windowing system distributor 'Microsoft Corp.', version 10.0.19043
System Description: Microsoft Windows 10 Home (v10.0.2009.19043.1503)

Configured using:
 'configure --with-json --without-dbus --with-native-compilation
 --without-compress-install CFLAGS=-O2'

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY W32NOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
XPM ZLIB

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search seq byte-opt
gv bytecomp byte-compile cconv mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils help-fns radix-tree cl-print
debug backtrace help-mode find-func time-date subr-x cl-loaddefs cl-lib
iso-transl tooltip eldoc paren electric uniquify ediff-hook vc-hooks
lisp-float-type elisp-mode mwheel dos-w32 ls-lisp disp-table
term/w32-win w32-win w32-vars term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite emoji-zwj charscript charprop case-table
epa-hook jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice
button loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads w32notify w32 lcms2 multi-tty
make-network-process native-compile emacs)

Memory information:
((conses 16 78996 5653)
 (symbols 48 7157 0)
 (strings 32 22876 2110)
 (string-bytes 1 755191)
 (vectors 16 14673)
 (vector-slots 8 308167 12600)
 (floats 8 30 232)
 (intervals 56 376 0)
 (buffers 992 11))