frontend: add BuildChroot(s) to Build ASAP if package is known
    
    When the build Package is known, it doesn't make any sense to delay the
    assignment of related BuildChroots till the source build is finished.
    Nothing can affect the set of BuildChroots, only the chroot denylist
    (already known) and potentially some project configuration change (we
    can afford this).
    
    This brings a bit more convenience to the monitor web-UI/api routes'
    users; users can observe the progress much sooner there (waiting state),
    and what's ore important - source build failures are visible there.
    Without this change, BuildChroot allocation is delayed to post-source
    build and there's a risk of source build failure causing that
    BuildChroots are not allocated at all (monitor can not show these
    Builds at all).
    
    This patch is trying to centralize the logic for BuildChroot allocation
    onto one place.