Fix sbt.nix to download artifacts from Maven/Ivy

[charleso]

In particular introduce a nix-prefetch-sbt command that download of Maven/Ivy artifacts from a set of mirrors. Look at nix-prefetch-url and co. for inspiration.

We should be using sbt:// URL prefix for everything, which is then handled by nix-prefetch-sbt. This is important so we never rely on jars being kept a specific location (and so they can be mirrored/cached if necessary).

We may want to create separate issues for different commands that help with dev lifecycle (eg testing), although I suspect the biggest win will be to implement an SBT resolver (see #7)

Finally, remove the file attribute on artifact.