Improved package command

[mmhat]

You can now include files that reside below the output directory in the package. This is for example useful if you want to include sub-packages in a top-level package, i.e:

> dhall package toplevel.dhall Sub/package.dhall Other/file.dhall
{ toplevel = ./toplevel.dhall
, Sub = ./Sub/package.dhall
, Other.file = ./Other/file.dhall
}