Make `getBaseWebpackConfig` synchronous. (#6356)

Currently, `getBaseWebpackConfig` is marked async, but it doesn't await anything or otherwise return a Promise.  Please correct me if I am wrong, but it looks like it can be made synchronous.

packages/next/build/index.ts

@@ -58,7 +58,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
   const pagePaths = await collectPages(pagesDir, config.pageExtensions)
   const pages = createPagesMapping(pagePaths, config.pageExtensions)
   const entrypoints = createEntrypoints(pages, config.target, buildId, config)
-  const configs: any = await Promise.all([
+  const configs: any = [
     getBaseWebpackConfig(dir, {
       buildId,
       isServer: false,
@@ -73,7 +73,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
       target: config.target,
       entrypoints: entrypoints.server,
     }),
-  ])
+  ]
 
   let result: CompilerResult = { warnings: [], errors: [] }
   if (config.target === 'serverless') {

packages/next/build/webpack-config.js

@@ -146,7 +146,7 @@ function optimizationConfig ({ dev, isServer, totalPages, target }) {
   return config
 }
 
-export default async function getBaseWebpackConfig (dir, { dev = false, isServer = false, buildId, config, target = 'server', entrypoints }) {
+export default function getBaseWebpackConfig (dir, { dev = false, isServer = false, buildId, config, target = 'server', entrypoints }) {
   const defaultLoaders = {
     babel: {
       loader: 'next-babel-loader',

packages/next/server/hot-reloader.js

@@ -168,10 +168,10 @@ export default class HotReloader {
     const pagePaths = await glob(`+(_app|_document).+(${this.config.pageExtensions.join('|')})`, { cwd: join(this.dir, 'pages') })
     const pages = createPagesMapping(pagePaths, this.config.pageExtensions)
     const entrypoints = createEntrypoints(pages, 'server', this.buildId, this.config)
-    return Promise.all([
+    return [
       getBaseWebpackConfig(this.dir, { dev: true, isServer: false, config: this.config, buildId: this.buildId, entrypoints: entrypoints.client }),
       getBaseWebpackConfig(this.dir, { dev: true, isServer: true, config: this.config, buildId: this.buildId, entrypoints: entrypoints.server })
-    ])
+    ]
   }
 
   async start () {