Runs cargo run --bin (without specifying a binary name) so that Cargo prints an error with
a list of available binary targets. Then parses that list to return a vector of Example instances,
using the provided prefix.
Deduplicates targets by their canonicalized origin.
In particular, if two targets share the same origin key and one is a SingleFile
and the other is a DefaultBinary, only the SingleFile target is kept.