Stories named index.stories.svelte are not recognized

Bug report reproduction repo. See storybookjs/storybook/issues/15295

(This repo is the output of running the storybook reproduction command as described here.)

I have historically set up my stories so that each component has a directory containing the component with the story right beside it. For example:

src/components
├── index.ts
└── Navbar
    ├── index.stories.svelte
    ├── index.svelte
    └── svelte-logo.svg

I found that index.stories.svelte is never picked up by Storybook. However, if I rename this file to, e.g., Navbar.stories.svelte then everything works fine.

Additional steps outside of this reproduction

In addition to the usual setup, I followed the guide described in #14929.