This makes a few refinements to bundling widgets:
* A widget with `published: false` is not shown in the
custom widget dropdown in the UI. This is so widgets
can be bundled with the app for "native" use (like the
calendar widget) without immediately resulting in an
extra listing in the UI. (There are improvements we'd
like to make to the UI to better communicate widget
provenance and quality eventually, which would be a
helpful alternative to just a binary flag.)
* A relative path to the custom widget manifest is
respected. This will make the bundling process marginally
neater.