resolve using pkg.svelte and pkg.svelte.root

[Rich-Harris]

This implements the first part of sveltejs/svelte#604 — if a component has a pkg.svelte property or a pkg['svelte.root'] property, this plugin will use that to locate the original uncompiled component, rather than the compiled distributable version.

Future work:

• put the resolution logic in a separate module so it can be used by other things like svelte-loader?
• Warn on version mismatches
• Respect pkg.engines