Don't bundle Svelte compiler

[Conduitry]

At least the latest alpha of Sapper is bundling the Svelte compiler (version 3.0.0-beta.3). This avoids issues with changes between compiler versions, but I don't think this is a good idea overall.

[Conduitry]

During the pre-release phase of Svelte and Sapper, I think it's reasonable to not be particularly careful about breaking changes or compatibility between different versions of the two projects. I don't foresee more changes in this area of Svelte, but if there are, people will just have to upgrade together.

[Conduitry]

It looks like the intention is to not bundle Svelte, but in rollup.config.js, we are constructing the external array from (in part) Object.keys(pkg.dependencies), which will contain 'svelte' but not the 'svelte/compiler' that Svelte 3 now uses.

[Rich-Harris]

blurgh. yeah, sounds like we need to update that Rollup config — definitely don't want to bundle the compiler