[Bug]: Storybook index crashes when directories match glob as stories

[yannbf]

When running vitest tests where test.browser.screenshotFailures is set to true, upon test failures a screenshot is stored in a path like src/components/__screenshots__/Button.stories.tsx/Primary.png.

Because there is a directory named Button.stories.tsx, it causes Storybooks indexer to crash.

We should probably handle directories as not stories, or at least add __screenshots__ as a default ignored path in the indexer.

More context: vitest-dev/vitest#6239