Right now the prorotype assumes that mapping of external validators to their executables is loaded into a hashtable, though no actual loading code exists and tests use a hashtable constructed by hand.
It seems like we don't really need it anyway, since a) being useless to end users, the validator executables have nothing to do in */bin dirs and something like /usr/libexec/vyos/validators seems to be the best place for them and b) validator definition files would contain just their names and paths to their executable, which makes them quite superflous.