Don't mutate argument to loadConfig

Summary:
Changelog: **[Fix]**: `metro-config`: Don't mutate argument to `loadConfig`.

This enables some improvements to type safety in how we define Metro's core CLI commands (coming up in a separate diff).

Reviewed By: jacdebug

Differential Revision: D43774991

fbshipit-source-id: 3f66f287e7d34764c8946a3a84483515dbf5d376

packages/metro-config/src/configTypes.flow.js

@@ -241,7 +241,7 @@ export type ConfigT = $ReadOnly<{
   }>,
 }>;
 
-export type YargArguments = {
+export type YargArguments = $ReadOnly<{
   config?: string,
   cwd?: string,
   port?: string | number,
@@ -259,4 +259,4 @@ export type YargArguments = {
   runInspectorProxy?: boolean,
   verbose?: boolean,
   ...
-};
+}>;

packages/metro-config/src/loadConfig.js

@@ -265,10 +265,10 @@ function overrideConfigWithArguments(
  * @return {object}                         Configuration returned
  */
 async function loadConfig(
-  argv?: YargArguments = {},
+  argvInput?: YargArguments = {},
   defaultConfigOverrides?: InputConfigT = {},
 ): Promise<ConfigT> {
-  argv.config = overrideArgument(argv.config);
+  const argv = {...argvInput, config: overrideArgument(argvInput.config)};
 
   const configuration = await loadMetroConfigFromDisk(
     argv.config,