fix: correctly resolve hooks entry file when directory exists

[eltigerchino]

fixes #13100

This PR adds a check to see if an index file exists in the related directory before resolving to that path. Directories without an index file should correctly fallback to the non-directory path.

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🦋 Changeset detected

Latest commit: fe771b2

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Patch

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13144-svelte.vercel.app/

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