toaster: eliminate duplicate querysets

toaster: eliminate duplicate querysets

The Layer_Version.get_equivalents_wpriority performs the same
function as Project.compatible_layerversions, but in memory and
with worse performance.

Replace the code in get_equivalents_wpriority with a call to
the project compatible_layerversions, which also returns a queryset
instead of a list (can be used to further enhance queries)

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>