fix: treat imports as potentially reactive

[Rich-Harris]

fixes #13212. supersedes #13264

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[trueadm]

I originally did this too, but @dummdidumm mentioned in the linked issue that only handling imports wasn't sufficient. Happy to go for either way. i.e. you did this:

<script>
  import * as foo from './stuff.svelte.js';

  const stuff = foo.stuff;
</script>

<button onclick={() => Object.assign(stuff, { x: 1 })}>1</button>
<button onclick={() => Object.assign(stuff, { x: 2 })}>2</button>
<p>{Object.values(stuff)}</p>

[Rich-Harris]

True. I think we should do #13266 so that we can preserve the optimal behaviour while maintaining correctness. Closing until then