chore(firefox): move sources from //lib/firefox to //lib (#3920)

experimental/puppeteer-firefox/lib/externs.d.ts

@@ -0,0 +1,28 @@
+import { Connection as RealConnection } from './Connection';
+import { Target as RealTarget } from './Browser';
+import * as child_process from 'child_process';
+declare global {
+  module Puppeteer {
+
+    export interface ConnectionTransport {
+      send(string);
+      close();
+      onmessage?: (message: string) => void,
+      onclose?: () => void,
+    }
+
+    export interface ChildProcess extends child_process.ChildProcess { }
+
+    export type Viewport = {
+      width: number;
+      height: number;
+      deviceScaleFactor?: number;
+      isMobile?: boolean;
+      isLandscape?: boolean;
+      hasTouch?: boolean;
+    }
+
+    export class Connection extends RealConnection { }
+    export class Target extends RealTarget { }
+  }
+}