Add flatpak- prefix to executables to avoid conflicting with xdg-utils
    
    This also makes sure we don't provide /usr/bin/xdg-open and
    /usr/bin/xdg-email as that can cause the wrong package
    (flatpak-xdg-utils) to be pulled in instead of regular xdg-utils on
    regular Fedora installs.
    
    This goes together with flatpak-runtime and flatpak-sdk changes to
    install flatpak-xdg-open and flatpak-xdg-email as /usr/bin/xdg-open
    and /usr/bin/xdg-email.