Handle job aliases in AggregateConstituents

Aggregate constituents are derivations.  However there can be multiple
builds in an evaluation that have the same derivation, i.e. they can
alias each other (e.g. "emacs", "emacs24" and "emacs24Packages.emacs"
in Nixpkgs).  Previously we picked a build arbitrarily for the
AggregateConstituents table.  Now we pick the one with the shortest
name (e.g. "emacs").