[flake8-builtins] Exempt private built-in modules (A005) (#14505)

## Summary

Resolves #12949.

## Test Plan

`cargo nextest run` and `cargo insta test`.

crates/ruff_linter/src/rules/flake8_builtins/mod.rs

@@ -36,6 +36,10 @@ mod tests {
         Rule::BuiltinModuleShadowing,
         Path::new("A005/modules/package/bisect.py")
     )]
+    #[test_case(
+        Rule::BuiltinModuleShadowing,
+        Path::new("A005/modules/_abc/__init__.py")
+    )]
     #[test_case(Rule::BuiltinModuleShadowing, Path::new("A005/modules/package/xml.py"))]
     #[test_case(Rule::BuiltinLambdaArgumentShadowing, Path::new("A006.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
@@ -91,6 +95,10 @@ mod tests {
         Rule::BuiltinModuleShadowing,
         Path::new("A005/modules/package/bisect.py")
     )]
+    #[test_case(
+        Rule::BuiltinModuleShadowing,
+        Path::new("A005/modules/_abc/__init__.py")
+    )]
     #[test_case(Rule::BuiltinModuleShadowing, Path::new("A005/modules/package/xml.py"))]
     fn builtins_allowed_modules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!(

crates/ruff_linter/src/rules/flake8_builtins/rules/builtin_module_shadowing.rs

@@ -1,14 +1,15 @@
 use std::path::Path;
 
-use crate::package::PackageRoot;
-use crate::settings::types::PythonVersion;
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::PySourceType;
 use ruff_python_stdlib::path::is_module_file;
 use ruff_python_stdlib::sys::is_known_standard_library;
 use ruff_text_size::TextRange;
 
+use crate::package::PackageRoot;
+use crate::settings::types::PythonVersion;
+
 /// ## What it does
 /// Checks for modules that use the same names as Python builtin modules.
 ///
@@ -47,25 +48,35 @@ pub(crate) fn builtin_module_shadowing(
         return None;
     }
 
-    if let Some(package) = package {
-        let module_name = if is_module_file(path) {
-            package.path().file_name().unwrap().to_string_lossy()
-        } else {
-            path.file_stem().unwrap().to_string_lossy()
-        };
+    let package = package?;
+
+    let module_name = if is_module_file(path) {
+        package.path().file_name().unwrap().to_string_lossy()
+    } else {
+        path.file_stem().unwrap().to_string_lossy()
+    };
+
+    if !is_known_standard_library(target_version.minor(), &module_name) {
+        return None;
+    }
+
+    // Shadowing private stdlib modules is okay.
+    // https://github.com/astral-sh/ruff/issues/12949
+    if module_name.starts_with('_') && !module_name.starts_with("__") {
+        return None;
+    }
 
-        if is_known_standard_library(target_version.minor(), &module_name)
-            && allowed_modules
-                .iter()
-                .all(|allowed_module| allowed_module != &module_name)
-        {
-            return Some(Diagnostic::new(
-                BuiltinModuleShadowing {
-                    name: module_name.to_string(),
-                },
-                TextRange::default(),
-            ));
-        }
+    if allowed_modules
+        .iter()
+        .any(|allowed_module| allowed_module == &module_name)
+    {
+        return None;
     }
-    None
+
+    Some(Diagnostic::new(
+        BuiltinModuleShadowing {
+            name: module_name.to_string(),
+        },
+        TextRange::default(),
+    ))
 }

crates/ruff_linter/src/rules/flake8_builtins/snapshots/ruff_linter__rules__flake8_builtins__tests__A005_A005__modules___abc____init__.py.snap

@@ -0,0 +1,5 @@
+---
+source: crates/ruff_linter/src/rules/flake8_builtins/mod.rs
+snapshot_kind: text
+---
+

crates/ruff_linter/src/rules/flake8_builtins/snapshots/ruff_linter__rules__flake8_builtins__tests__A005_A005__modules___abc____init__.py_builtins_allowed_modules.snap

@@ -0,0 +1,5 @@
+---
+source: crates/ruff_linter/src/rules/flake8_builtins/mod.rs
+snapshot_kind: text
+---
+