Move some utils to clippy_utils::source module

clippy_lints/src/default_numeric_fallback.rs

@@ -1,3 +1,5 @@
+use clippy_utils::source::snippet;
+use if_chain::if_chain;
 use rustc_ast::ast::{LitFloatType, LitIntType, LitKind};
 use rustc_errors::Applicability;
 use rustc_hir::{
@@ -11,9 +13,7 @@ use rustc_middle::{
 };
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
-use if_chain::if_chain;
-
-use crate::utils::{snippet, span_lint_and_sugg};
+use crate::utils::span_lint_and_sugg;
 
 declare_clippy_lint! {
     /// **What it does:** Checks for usage of unconstrained numeric literals which may cause default numeric fallback in type