move dsl::exists into default feature

Author: vnghia

diesel/src/expression/mod.rs

@@ -26,10 +26,7 @@ pub(crate) mod assume_not_null;
 pub(crate) mod bound;
 mod coerce;
 pub(crate) mod count;
-#[cfg(not(feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes"))]
 pub(crate) mod exists;
-#[cfg(feature = "i-implement-a-third-party-backend-and-opt-into-breaking-changes")]
-pub mod exists;
 pub(crate) mod grouped;
 pub(crate) mod helper_types;
 mod not;