fix: only try to load external files with relative paths

[kaisermann]

Closes sveltejs/kit#4062

The isValidLocalPath was checking the path validity the other way around (not remote). We should instead check if it's a valid local path (starts with . and doesn't contain interpolation).

[dummdidumm]

Makes sense to me. I thought about whether or not this can be seen as a breaking change, but the existing behavior was unexpected, too, and the resolved file path in the /app.js case likely was wrong anyway, so I concluded that this is just a fix 👍