Push packaging down to shoes-swt

Packaging was previously baked into the `Shoes::CLI` class, which lived up
in `shoes` (and now in `shoes-core`). We don't want JRuby-specific dependencies
at that level, though, so this belonged down in `shoes-swt`.

We accomplish this by creating a DSL and backend class to represent
packaging, and call those from the CLI. This lets us move the `furoshiki`
dependency down to `shoes-swt` as well... extra win!