Incorrect delegation lookups can make go-tuf download the wrong artifact

go-tuf is a Go implementation of The Update Framework (TUF). The go-tuf client inconsistently traces the delegations. For example, if targets delegate to "A", and to "B", and "B" delegates to "C", then the client should trace the delegations in the order "A" then "B" then "C" but it may incorrectly trace the delegations "B"->"C"->"A". This vulnerability is fixed in 2.0.1.