Allow sys.path modifications between imports (#9047)

## Summary

It's common to interleave a `sys.path` modification between imports at
the top of a file. This is a frequent cause of `# noqa: E402` false
positives, as seen in the ecosystem checks. This PR modifies E402 to
omit such modifications when determining the "import boundary".

(We could consider linting against `sys.path` modifications, but that
should be a separate rule.)

Closes: https://github.com/astral-sh/ruff/issues/5557.

crates/ruff_python_semantic/src/analyze/imports.rs

@@ -0,0 +1,37 @@
+use ruff_python_ast::{self as ast, Expr, Stmt};
+
+use crate::SemanticModel;
+
+/// Returns `true` if a [`Stmt`] is a `sys.path` modification, as in:
+/// ```python
+/// import sys
+///
+/// sys.path.append("../")
+/// ```
+pub fn is_sys_path_modification(stmt: &Stmt, semantic: &SemanticModel) -> bool {
+    let Stmt::Expr(ast::StmtExpr { value, range: _ }) = stmt else {
+        return false;
+    };
+    let Expr::Call(ast::ExprCall { func, .. }) = value.as_ref() else {
+        return false;
+    };
+    semantic
+        .resolve_call_path(func.as_ref())
+        .is_some_and(|call_path| {
+            matches!(
+                call_path.as_slice(),
+                [
+                    "sys",
+                    "path",
+                    "append"
+                        | "insert"
+                        | "extend"
+                        | "remove"
+                        | "pop"
+                        | "clear"
+                        | "reverse"
+                        | "sort"
+                ]
+            )
+        })
+}

crates/ruff_python_semantic/src/analyze/mod.rs

@@ -1,4 +1,5 @@
 pub mod function_type;
+pub mod imports;
 pub mod logging;
 pub mod type_inference;
 pub mod typing;