tree: use trie in DvcTree (or in Repo)

[efiop]

We are only using outputs there and we have the metadata now, so it makes sense to just construct the trie from the repo dag and have a nice and convenient way of working with it internally instead of doing dag lookups. Potentially worth doing the same thing in the repo itself, as trie is often handier than dag (though we are already using the tree(not trie) in such places). One repo trie with both outs and deps might be a solution too.

This became relevant while working on abstracting dir_info away into itsown class and analysing the way we do DvcTree.walk() now (we use trie there already).