Add migrate command

[f-f]

This PR adds a migrate command, to convert existing Dhall configurations to the new-style YAML configurations that the next-version (codenamed "spaghetto") will be using.

The command tries to read in the URL of the package set, converts it to a pointer to the JSON version (that Spaghetto can read), and dumps that and the rest of the configuration into a fresh configuration that can be used for the build.

A few notes:

• there's a test that checks the generated config against a fixture, but we're not checking if the config works with the new Spago. This is mostly because the config format is not yet set in stone - we should wait until some users try the first alpha of Spaghetto before committing to this
• we issue a warning about the fact that we are not converting user's overrides to the package set. This is because doing so is a little involved, and I'm focused on getting Spaghetto's alpha off the ground at the moment - however I left a few pointers in the source so if anyone's interested in implementing this I can provide guidance
• right now we are converting the url of the package set to another url - however it would be way better (for cache-hits reasons, as we'd fetch things from the registry instead of cloning repos) to use the package sets from the registry (this is done by using the syntax registry: 0.1.0 instead of e.g. url: https://... in the set: section). We should probably add a flag along the lines of --use-registry-set for folks that want to opt into the new sets.