Fix: preserve ./netlify/functions-internal

[lemusthelroy]

Summary

Some Netlify integrations, such as the emails integration, auto generate functions prior to the build and these will exist in the functions-internal folder. I am not totally aware of why the functions-internal directory is emptied currently by this adapter, however, this PR ensures that only the files that are generated by SvelteKit are removed and not any other files.

How has this been tested?

• With a new svelte kit project, that has the Netlify adapter added and running netlify build
• Adapting the above project, setting split:true and running netlify build again
• Reversing the above setting and running netlify build again

Closes #8903

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[benmccann]

I am not totally aware of why the functions-internal directory is emptied currently by this adapter, however, this PR ensures that only the files that are generated by SvelteKit are removed and not any other files.

It was just done to remove any leftover files from previous builds

[lemusthelroy]

I have just spotted that the name of the files generated could be dynamic - https://github.com/sveltejs/kit/pull/8899/files#diff-cc76725f3c6f8ff40fb8605aff3b0d3eeefc561e4a6c9a162b79dda99c95dedaL196.

[dummdidumm]

Thank you! Could you add a changeset? Then we're good to merge.

[orinokai]

Thanks Lewis. All looks good 🎉

[lemusthelroy]

Thank you! Could you add a changeset? Then we're good to merge.

Added 👍