Use existing //# sourceURL comments if present (#706)

With Wrangler's new `find_additional_modules` support, there are
cases where we'd like `//# sourceURL` not to be resolved from the
module name/path. This change makes sure we don't insert this
comment if it's already present, so Wrangler can add it.

packages/miniflare/src/plugins/core/modules.ts

@@ -134,6 +134,10 @@ function moduleName(modulesRoot: string, modulePath: string) {
 	return path.sep === "\\" ? name.replaceAll("\\", "/") : name;
 }
 export function withSourceURL(script: string, scriptPath: string): string {
+	// If we've already got a `//# sourceURL` comment, return `script` as is
+	// (searching from the end as that's where we'd expect it)
+	if (script.lastIndexOf("//# sourceURL=") !== -1) return script;
+
 	let scriptURL: URL | string = scriptPath;
 	if (maybeGetStringScriptPathIndex(scriptPath) === undefined) {
 		scriptURL = pathToFileURL(scriptPath);