Move extern crates to the root

crates/lint-policy-cli/src/diff.rs

@@ -1,9 +1,7 @@
 //! Diffing lint.
 
-use core::cmp::Ordering;
-
-extern crate alloc;
 use alloc::collections::BTreeSet;
+use core::cmp::Ordering;
 
 use lint_policy::cargo::{LintSetting, LintTable};
 

crates/lint-policy-cli/src/group.rs

@@ -1,6 +1,5 @@
 //! Lint groups.
 
-extern crate alloc;
 use alloc::collections::BTreeMap;
 
 use lint_policy::Linter;

crates/lint-policy-cli/src/main.rs

@@ -2,6 +2,8 @@
 
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
+extern crate alloc;
+
 use anyhow::Result;
 use clap::Parser as _;
 use lint_policy::Linter;