Improve the way shot-scraper har -x -o out/ works

[simonw]

Manual testing shows the interaction with -o is a bit confusing:

uv run --with-editable '.[test]' shot-scraper har datasette.io -o /tmp/wumpit.har -x

/tmp % ls -lah wumpit.har  
-rw-r--r--  1 simon  wheel  160766 Dec 28 12:33 wumpit.har
/tmp % rm wumpit.har 
/tmp % ls wumpit
datasette-io-static-datasette-logo.svg
datasette-io-static-lite-yt-embed.css
datasette-io-static-lite-yt-embed.js
...

Originally posted by @simonw in #183 (comment)

[simonw]

I think:

shot-scraper har datasette.io -x -o /tmp/datasette

Should create /tmp/datasette.har and also fill /tmp/datasette/ with the extracted HAR files.