fix: speed up build by ensuring stable cache

[dummdidumm]

Our builds got slower because the cache hash was always different. This happened because the dependencies were calculated based on the generated code after Vite compiled it, i.e. it would check stuff in .svelte-kit/output/server/chunks/...js. This seems to have worked fine for a while, but now Vite mashes things up such that code from svelte.dev/src/lib/server/content.ts is mixed in, which has loads of Vite hashes in it, and so the file content is always different. That in turn means our hash is always different, and so the snippet cache never hits.

The pragmatic solution is to hard-code the path to the original file and add a sanity check to ensure it's still accurate.

related to #307

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[dummdidumm]

ok so the problem is that for some reason Vite mashes things up in a way that messes with the import graph, and so src/lib/server/content.ts is mashed up with utils stuff. And because content.js includes all the imports to our content, it's always different and then no match. We need to somehow tell it to use the original files.

[Rich-Harris]

edited the comment to remove 'closes' — would like to keep #307 open as even with the cache it feels like things could be a hell of a lot faster. It definitely shouldn't take over a minute to render a few pages of markdown

[dummdidumm]

In my local testing it barely took 50 seconds once the cache was properly in place