[fix] do not warn if module variables are not the only dependencies in reactive statements

[tanhauhau]

#5847 warn that module variables are not reactive when used in a reactive statement

<script context="module">
   let PI = 3.14;
</script>
<script>
  $: twoPi = 2 * PI;
</script>

however this is a valid use case, which should not have warnings:

<script context="module">
   let PI = 3.14;
</script>
<script>
  let r;
  $: area = PI * r * r;
</script>

Before submitting the PR, please make sure you do the following

• [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint