Re-export metro-config's resolveConfig from metro

Summary:
`react-native-community/cli-plugin-metro` has a dependency on `mergeConfig`, `loadConfig` and `resolveConfig` from `metro-config`, here:

https://github.com/react-native-community/cli/blob/v12.0.0-alpha.2/packages/cli-plugin-metro/src/tools/loadMetroConfig.ts#L3-L9

Otherwise, `cli-plugin-metro` only has build-time dependencies on `metro-config`. By exporting `resolveConfig` in addition to the other two (which we already re-export), we can remove the `metro-config` runtime dependency and use `metro` instead.

That's a step closer to making `metro` a `peerDependency` of `cli-plugin-metro`, with version dictated by the host project's dependency, ultimately decoupling Metro from the CLI.

Changelog:
```
* **[Feature]** Re-export `metro-config`'s `resolveConfig` from `metro`.
```

Reviewed By: hoxyq

Differential Revision: D44870484

fbshipit-source-id: c38175cac5904b0b9540bab6b4c8e70d957d378e

packages/metro/src/index.flow.js

@@ -38,7 +38,12 @@ const chalk = require('chalk');
 const fs = require('fs');
 const http = require('http');
 const https = require('https');
-const {getDefaultConfig, loadConfig, mergeConfig} = require('metro-config');
+const {
+  getDefaultConfig,
+  loadConfig,
+  mergeConfig,
+  resolveConfig,
+} = require('metro-config');
 const {InspectorProxy} = require('metro-inspector-proxy');
 const net = require('net');
 const {parse} = require('url');
@@ -174,6 +179,7 @@ async function runMetro(
 exports.runMetro = runMetro;
 exports.loadConfig = loadConfig;
 exports.mergeConfig = mergeConfig;
+exports.resolveConfig = resolveConfig;
 
 const createConnectMiddleware = async function (
   config: ConfigT,

packages/metro/types/index.d.ts

@@ -19,7 +19,6 @@ import type {Server as HttpsServer} from 'https';
 import type {
   ConfigT,
   InputConfigT,
-  loadConfig,
   MetroConfig,
   Middleware,
 } from 'metro-config';
@@ -30,6 +29,8 @@ import Yargs = require('yargs');
 import type {default as MetroServer, ServerOptions} from './Server';
 import type {OutputOptions, RequestOptions} from './shared/types';
 
+export {loadConfig, mergeConfig, resolveConfig} from 'metro-config';
+
 export {HttpServer, HttpsServer};
 
 interface MetroMiddleWare {
@@ -114,8 +115,6 @@ export function runMetro(
   options?: RunMetroOptions,
 ): Promise<MetroServer>;
 
-export {loadConfig};
-
 export function createConnectMiddleWare(
   config: ConfigT,
   options?: RunMetroOptions,