notmuch: make emacs a separate output

This puts all emacs-related binaries and code to a separate output,
removing emacs from the runtime closure of neomutt.

nixos/doc/manual/release-notes/rl-2009.xml

@@ -215,7 +215,13 @@ environment.systemPackages = [
 
   <itemizedlist>
    <listitem>
-    <para />
+    <para>
+     The <package>notmuch</package> package move its emacs-related binaries and
+     emacs lisp files to a separate output. They're not part
+     of the default <literal>out<literal> output anymore - if you relied on the
+     <literal>notmuch-emacs-mua</literal> binary or the emacs lisp files, access them via
+     the <literal>notmuch.emacs</literal> output.
+    </para>
    </listitem>
   </itemizedlist>
  </section>

pkgs/applications/networking/mailreaders/notmuch/default.nix

@@ -58,6 +58,7 @@ stdenv.mkDerivation rec {
     "--zshcompletiondir=${placeholder "out"}/share/zsh/site-functions"
     "--infodir=${placeholder "info"}"
   ] ++ optional (!withEmacs) "--without-emacs"
+    ++ optional (withEmacs) "--emacslispdir=${placeholder "emacs"}/share/emacs/site-lisp"
     ++ optional (isNull ruby) "--without-ruby";
 
   # Notmuch doesn't use autoconf and consequently doesn't tag --bindir and
@@ -67,7 +68,7 @@ stdenv.mkDerivation rec {
   makeFlags = [ "V=1" ];
 
 
-  outputs = [ "out" "man" "info" ];
+  outputs = [ "out" "man" "info" ] ++ stdenv.lib.optional withEmacs "emacs";
 
   preCheck = let
     test-database = fetchurl {
@@ -86,6 +87,10 @@ stdenv.mkDerivation rec {
 
   installTargets = [ "install" "install-man" "install-info" ];
 
+  postInstall = stdenv.lib.optionalString withEmacs ''
+    moveToOutput bin/notmuch-emacs-mua $emacs
+  '';
+
   dontGzipMan = true; # already compressed
 
   meta = {