Cache transformations based on config as well as file contents

[Rich-Harris]

Hi! I've been working on a Snowpack/SSR proof of concept, and have run into what appears to be a bug. In that repo, there are two instances of Snowpack running — one that compiles Svelte components in ssr mode, and one that compiles them in dom mode.

Because the caching is based on the file name and contents, when the browser loads the server-rendered page generated from (say) App.svelte, and subsequently loads the dom version of App.svelte to hydrate the server-rendered markup, Snowpack responds with the ssr version instead because that's what's cached. (Oddly enough it seems to fix itself if you request the file enough times.)

It feels like we need to somehow factor Snowpack's configuration into the caching, so that the same file can be transformed differently by different Snowpack instances.