pulseaudioFull: fix wrapGApp wrapping

Since

7a2605e0f3

the pulseaudio build recipe incorporates the `wrapGAppsHook`
wrapper setup-hook if `advancedBluetoothCodecs` are enabled.
This wrapper setup-hook -- like most wrappers --
wraps binaries in `$out/bin` by first renaming them,
then placing a wrapper script where the original binary was.

Unfortunatelly, pulseaudio doesn't like its
binary moved around after installation:
It records the binaries path during installation time

e5ad31e873/meson.build (L154)

then checks the path in `/proc/self/exe` and complains

> Jun 16 19:06:48 nixosb pulseaudio[2219]: W: [.pulseaudio-wra] main.c: /proc/self/exe does not point to /nix/store/bqfyzxwpxa2ydmyvh3j32xrm4chxbj22-pulseaudio-15.0/bin/pulseaudio, cannot self execute. Are you playing games?

if they don't match

e5ad31e873/src/daemon/main.c (L577)

Somehow, this also results in a real bug:
`pacmd` fails to connect to the pulseaudio server, see

https://github.com/NixOS/nixpkgs/issues/177915

To fix this issue, the commit at hand changes the
installation directory for binaries to `$out/.bin-unwrapped`.
After the installation, `$out/bin` is created by hand and
populated with symlinks to files in `$out/.bin-unwrapped`.
`wrapGAppsHook` doesn't know or care about the `.bin-unwrapped`
directory; it just sees all the symlinks in `bin`,
renames them and places wrapper scripts beside them.

Effectively, this leaves the original
binary in `.bin-unwrapped` unchanged!
So pulseaudio will find itself still in its oritinal place,
and "users" of the package can call pulseaudio
via the wrapper script in `bin` as usual.

pkgs/servers/pulseaudio/default.nix

@@ -116,6 +116,11 @@ stdenv.mkDerivation rec {
     "-Dsysconfdir=/etc"
     "-Dsysconfdir_install=${placeholder "out"}/etc"
     "-Dudevrulesdir=${placeholder "out"}/lib/udev/rules.d"
+
+    # pulseaudio complains if its binary is moved after installation;
+    # this is needed so that wrapGApp can operate *without*
+    # renaming the unwrapped binaries (see below)
+    "--bindir=${placeholder "out"}/.bin-unwrapped"
   ]
   ++ lib.optional (stdenv.isLinux && useSystemd) "-Dsystemduserunitdir=${placeholder "out"}/lib/systemd/user"
   ++ lib.optionals stdenv.isDarwin [
@@ -133,11 +138,11 @@ stdenv.mkDerivation rec {
   postInstall = lib.optionalString libOnly ''
     find $out/share -maxdepth 1 -mindepth 1 ! -name "vala" -prune -exec rm -r {} \;
     find $out/share/vala -maxdepth 1 -mindepth 1 ! -name "vapi" -prune -exec rm -r {} \;
-    rm -r $out/{bin,etc,lib/pulse-*}
+    rm -r $out/{.bin-unwrapped,etc,lib/pulse-*}
   ''
     + ''
     moveToOutput lib/cmake "$dev"
-    rm -f $out/bin/qpaeq # this is packaged by the "qpaeq" package now, because of missing deps
+    rm -f $out/.bin-unwrapped/qpaeq # this is packaged by the "qpaeq" package now, because of missing deps
   '';
 
   preFixup = lib.optionalString (stdenv.isLinux  && (stdenv.hostPlatform == stdenv.buildPlatform)) ''
@@ -151,6 +156,15 @@ stdenv.mkDerivation rec {
       ln -s "''$file" "''${file%.dylib}.so"
       ln -s "''$file" "$out/lib/pulseaudio/''$(basename ''$file .dylib).so"
     done
+  ''
+  # put symlinks to binaries in `$prefix/bin`;
+  # then wrapGApp will *rename these symlinks* instead of
+  # the original binaries in `$prefix/.bin-unwrapped` (see above);
+  # when pulseaudio is looking for its own binary (it does!),
+  # it will be happy to find it in its original installation location
+  + lib.optionalString (!libOnly) ''
+    mkdir -p $out/bin
+    ln -st $out/bin $out/.bin-unwrapped/*
   '';
 
   passthru = {