refactor: simplify internal compilers

lib/builder/builder.js

@@ -30,7 +30,7 @@ export default class Builder {
     this.options = nuxt.options
 
     // Fields that set on build
-    this.compiler = null
+    this.compilers = []
     this.webpackDevMiddleware = null
     this.webpackHotMiddleware = null
 
@@ -408,23 +408,19 @@ export default class Builder {
       compilersOptions.push(dllWebpackConfig.call(this, clientConfig))
     }
 
-    // Simulate webpack multi compiler interface
-    // Separate compilers are simpler, safer and faster
-    this.compilers = []
-
     // Initialize shared FS and Cache
     const sharedFS = this.options.dev && new MFS()
     const sharedCache = {}
 
     // Initialize compilers
-    compilersOptions.forEach(compilersOption => {
+    this.compilers = compilersOptions.map(compilersOption => {
       const compiler = webpack(compilersOption)
       // In dev, write files in memory FS (except for DLL)
       if (sharedFS && !compiler.name.includes('-dll')) {
         compiler.outputFileSystem = sharedFS
       }
       compiler.cache = sharedCache
-      this.compilers.push(compiler)
+      return compiler
     })
 
     // Start Builds