emacs: make trivialBuild know its elisp dependencies in another way

Previously, trivialBuild did not know how to find its elisp
dependencies.  This was[1] fixed[2] by basically rewriting part of
package-activate-all in bash.

I think it is better to call package-activate-all (or
package-initialize if Emacs is old) directly.  It reduces maintenance
burden a bit.  It also improves consistency since elpaBuild and
melpaBuild already do so.

This change provides almost the same functionality as before.  It only
breaks elisp packages with non-standard[^3] elisp dependencies.
However, I think those non-standard ones should be fixed instead.
As an example, mu4e used to be a non-standard one and was fixed[4].

This change does not cause more build failures in emacsPackages.

[1]: https://github.com/NixOS/nixpkgs/pull/82604
[2]: https://github.com/NixOS/nixpkgs/commit/bf486f784ddd969c03243dba4c93d0e8e861173e
[^3]: Non-standard elisp packages do not meet requirements of
package.el, the builtin package manager of Emacs.  Usually, they are
installed to $out/share/emacs/site-lisp/$pname-$version and/or miss a
$pname-pkg.el file.
[4]: https://github.com/NixOS/nixpkgs/pull/253438

pkgs/applications/editors/emacs/build-support/emacs-funcs.sh

@@ -20,15 +20,4 @@ addEmacsVars () {
   if [ -n "${addEmacsNativeLoadPath:-}" ]; then
     addToEmacsNativeLoadPath "$1/share/emacs/native-lisp"
   fi
-
-  # Add sub paths to the Emacs load path if it is a directory
-  # containing .el files. This is necessary to build some packages,
-  # e.g., using trivialBuild.
-  for lispDir in \
-      "$1/share/emacs/site-lisp/"* \
-      "$1/share/emacs/site-lisp/elpa/"*; do
-    if [[ -d $lispDir && "$(echo "$lispDir"/*.el)" ]] ; then
-      addToEmacsLoadPath "$lispDir"
-    fi
-  done
 }

pkgs/applications/editors/emacs/build-support/generic.nix

@@ -86,10 +86,14 @@ libBuildHelper.extendMkDerivation' stdenv.mkDerivation (finalAttrs:
     source ${./emacs-funcs.sh}
     addEmacsVars "$out"
 
+    # package-activate-all is used to activate packages.  In other builder
+    # helpers, package-initialize is used for this purpose because
+    # package-activate-all is not available before Emacs 27.
     find $out/share/emacs -type f -name '*.el' -not -name ".dir-locals.el" -print0 \
       | xargs --verbose -0 -I {} -n 1 -P $NIX_BUILD_CORES sh -c \
           "emacs \
              --batch \
+             -f package-activate-all \
              --eval '(setq native-comp-eln-load-path (cdr native-comp-eln-load-path))' \
              --eval '(setq large-file-warning-threshold nil)' \
              --eval '(setq byte-compile-error-on-warn ${if finalAttrs.turnCompilationWarningToError then "t" else "nil"})' \

pkgs/applications/editors/emacs/build-support/trivial.nix

@@ -19,7 +19,7 @@ args:
       foundMakefile=1
     fi
 
-    emacs -L . --batch -f batch-byte-compile *.el
+    emacs -l package -f package-initialize -L . --batch -f batch-byte-compile *.el
 
     runHook postBuild
   '';