Merge pull request #9492 from storybookjs/9490-fix-source-loader-imports

[shilman]

Issue: #9490

What I did

Always insert the preamble code at the beginning of the file. See the repro in #9490

import A1
export A2
import B1
-> (inserted code)
export B2

=>

-> (inserted code)
import A1
export A2
import B1
export B2

How to test

See new official-storybook example, which fails before the change & succeeds after

[vercel]

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[tmeasday]

LGTM based on discussion