Fix Gradle task to properly detect Smithy model changes

The generateSmithyBuild task was not correctly declaring Smithy model files
as inputs, causing it to not regenerate when models changed. This fixes
the path resolution and adds an existence check for the input files.

Author: rcoh

buildSrc/src/main/kotlin/CodegenTestCommon.kt

@@ -236,8 +236,13 @@ fun Project.registerGenerateSmithyBuildTask(
     this.tasks.register("generateSmithyBuild") {
         description = "generate smithy-build.json"
         outputs.file(project.projectDir.resolve("smithy-build.json"))
-        // NOTE: This is not working.
-        allCodegenTests.flatMap { it.imports }.forEach { inputs.file(project.projectDir.resolve(it)) }
+        // Declare Smithy model files as inputs so task reruns when they change
+        allCodegenTests.flatMap { it.imports }.forEach {
+            val resolvedPath = project.projectDir.resolve(it)
+            if (resolvedPath.exists()) {
+                inputs.file(resolvedPath)
+            }
+        }
 
         doFirst {
             project.projectDir.resolve("smithy-build.json")