How to install Pantheon Desktop on Ubuntu 15.04?

Question

Yeah none of the other solutions in How to install the Pantheon desktop environment? for older versions of Ubuntu worked out. The only ppa that wouldn't say "404 not found" when updated was the ppa:elementary-os/daily, but I couldn't install elementary-desktop, since it just said (in the console) it wouldn't install packages related with it.

The following packages have unmet dependencies:
 elementary-desktop : Depends: ubuntu-extras-keyring but it is not installed
                      Recommends: apport-hooks-elementary but it is not going to be installed
                      Recommends: bluez-gstreamer but it is not going to be installed
                      Recommends: elementary-default-settings but it is not going to be installed
                      Recommends: elementary-os-prerelease but it is not going to be installed
                      Recommends: fonts-elementary-core but it is not going to be installed
                      Recommends: gcalctool but it is not going to be installed
                      Recommends: geary but it is not going to be installed
                      Recommends: ginn but it is not going to be installed
                      Recommends: gtksourceview-3.0-theme-solarized-elementary but it is not installed
                      Recommends: ibus-pinyin
                      Recommends: light-locker but it is not going to be installed
                      Recommends: pantheon-greeter but it is not going to be installed
                      Recommends: plymouth-theme-elementary but it is not going to be installed
                      Recommends: plymouth-theme-elementary-text but it is not going to be installed
                      Recommends: print-test-page-elementary but it is not installed
                      Recommends: pulseaudio-module-gconf but it is not going to be installed
                      Recommends: switchboard but it is not going to be installed
                      Recommends: switchboard-gnome-control-center-override but it is not going to be installed
                      Recommends: switchboard-plug-about but it is not going to be installed
                      Recommends: switchboard-plug-applications but it is not going to be installed
                      Recommends: switchboard-plug-datetime but it is not going to be installed
                      Recommends: switchboard-plug-display but it is not going to be installed
                      Recommends: switchboard-plug-gcc-bluetooth but it is not going to be installed
                      Recommends: switchboard-plug-gcc-color but it is not going to be installed
                      Recommends: switchboard-plug-gcc-mouse but it is not going to be installed
                      Recommends: switchboard-plug-gcc-network but it is not going to be installed
                      Recommends: switchboard-plug-gcc-printer but it is not going to be installed
                      Recommends: switchboard-plug-gcc-sound but it is not going to be installed
                      Recommends: switchboard-plug-gcc-ua but it is not going to be installed
                      Recommends: switchboard-plug-gcc-users but it is not going to be installed
                      Recommends: switchboard-plug-gcc-wacom but it is not going to be installed
                      Recommends: switchboard-plug-keyboard but it is not going to be installed
                      Recommends: switchboard-plug-locale but it is not going to be installed
                      Recommends: switchboard-plug-notifications but it is not installed
                      Recommends: switchboard-plug-online-accounts but it is not going to be installed
                      Recommends: switchboard-plug-pantheon-shell but it is not going to be installed
                      Recommends: switchboard-plug-power but it is not going to be installed
                      Recommends: switchboard-plug-security-privacy but it is not going to be installed
                      Recommends: ttf-punjabi-fonts but it is not going to be installed
                      Recommends: tumbler-plugins-extra but it is not going to be installed
                      Recommends: user-specific-alternatives but it is not going to be installed
E: Unable to correct problems, you have held broken packages.  

Answer 1

I've managed to install elementary desktop by rebuilding ubuntu-extras-keyring locally on my system, like this:

mkdir somedir; cd somedir
sudo apt-get install -y debhelper
wget http://archive.ubuntu.com/ubuntu/pool/main/u/ubuntu-extras-keyring/ubuntu-extras-keyring_2010.09.27.{tar.gz,dsc}
tar -xf *.tar.gz
cd ubuntu-extras-keyring-2010.09.17
debuild -i -us -uc -b
sudo dpkg -i ../ubuntu-extras-keyring_2010.09.27_all.deb

After that, elementary-desktop will install, but will error out at the end because it depends on systemd-shim which is no longer needed nor wanted with 15.04 having a full SystemD implementation.

When it errors out, run

sudo dpkg --force-depends -r indicator-datetime
sudo apt-get purge unity-control-center unity-control-center-signon

and it should complete the installation.

I expect in the near future the Elementary folks will fix their repository to handle SystemD properly - Ubuntu 15.04 move to SystemD was done a few weeks before the official release and the Elementary guys probably didn't have time to adapt to that.

After installation, you can log out and see "Pantheon" in your display manager's session list, but if you try to log in you'd get just the blueprint paper styled Elementary "beta wallpaper". The problem is that the session starts Slingshot and Gala, but not WingPanel or Plank. Currently I'm using using Slingshot's ALT-F2 shortcut to open the launcher and run Plank and WingPanel manually (just type the name in all lowercase and hit ENTER).