Fix upgrade error when using @import … source(…)

[RobinMalfait]

This PR fixes an issue when running the upgrade tool and if any of the CSS files has an import that looks like this:

@import "tailwindcss" source("…");

This was trying to resolve tailwindcss" source("… instead of tailwindcss.

This PR fixes that by fixing the regex we used. Made the quantifier lazy to match as few characters as possible and thus only matching tailwindcss in this case.

Test plan

1. Ran it locally on a project

Before:

After: