import serde on all files

Author: thomas-k-cameron

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/rustlang/RustWriter.kt

@@ -454,9 +454,7 @@ class RustWriter private constructor(
         fun forModule(module: String?): RustWriter = if (module == null) {
             RustWriter("lib.rs", "crate")
         } else {
-            var writer = RustWriter("$module.rs", "crate::$module")
-            RenderSerdeAttribute.importSerde(writer)
-            writer
+            RustWriter("$module.rs", "crate::$module")
         }
 
         fun factory(debugMode: Boolean): Factory<RustWriter> = Factory { fileName: String, namespace: String ->
@@ -517,6 +515,7 @@ class RustWriter private constructor(
     init {
         expressionStart = '#'
         if (filename.endsWith(".rs")) {
+            RenderSerdeAttribute.importSerde(this)
             require(namespace.startsWith("crate") || filename.startsWith("tests/")) {
                 "We can only write into files in the crate (got $namespace)"
             }