Split checks and plugins into source-related modules (#447)

src/flake8_builtins/checks.rs

@@ -0,0 +1,20 @@
+use crate::ast::types::Range;
+use crate::checks::{Check, CheckKind};
+use crate::flake8_builtins::types::ShadowingType;
+use crate::python::builtins::BUILTINS;
+
+/// Check builtin name shadowing.
+pub fn builtin_shadowing(name: &str, location: Range, node_type: ShadowingType) -> Option<Check> {
+    if BUILTINS.contains(&name) {
+        Some(Check::new(
+            match node_type {
+                ShadowingType::Variable => CheckKind::BuiltinVariableShadowing(name.to_string()),
+                ShadowingType::Argument => CheckKind::BuiltinArgumentShadowing(name.to_string()),
+                ShadowingType::Attribute => CheckKind::BuiltinAttributeShadowing(name.to_string()),
+            },
+            location,
+        ))
+    } else {
+        None
+    }
+}

src/flake8_builtins/mod.rs

@@ -0,0 +1,2 @@
+pub mod checks;
+pub mod types;

src/flake8_builtins/types.rs

@@ -0,0 +1,5 @@
+pub enum ShadowingType {
+    Variable,
+    Argument,
+    Attribute,
+}