Dynamic ordering of resolvers

[maxnorth]

There can be dependencies between resolvers. For instance, if resolver A needs to use an env var that is a secret resolved by B, nv needs to make sure all values resolved by B run first. Users should be able to configure which env vars a resolver uses, so nv can resolve them first if they are secret refs. In some cases, it may be possible to automatically identify env vars a resolver depends on too. nv will also need to account for the possibility of circular dependencies.