ui2: Reference theme2 and settings2 crates without the 2 (#3424)

This PR updates the `ui2` crate to reference the `theme2` and
`settings2` crates without the `2` the way we do in our other crates.

Release Notes:

- N/A

crates/ui2/src/components/checkbox.rs

@@ -1,7 +1,6 @@
 use gpui::{div, prelude::*, Div, Element, ElementId, IntoElement, Styled, WindowContext};
 
-use theme2::ActiveTheme;
-
+use crate::prelude::*;
 use crate::{Color, Icon, IconElement, Selection};
 
 pub type CheckHandler = Box<dyn Fn(&Selection, &mut WindowContext) + 'static>;