feat: Introduce BrowserFetcher class (#1983)

This patch introduces `BrowserFetcher` class that manages
downloaded versions of products.

This patch:
- shapes Downloader API to be minimal yet usable for our needs. This
  includes removing such methods as `Downloader.supportedPlatforms` and
  `Downloader.defaultRevision`.
- makes most of the fs-related methods in Downloader async. The only
  exception is the `Downloader.revisionInfo`: it has stay sync due to the
  `pptr.executablePath()` method being sync.
- updates `install.js` and `utils/check_availability.js` to use new API
- finally, renames `Downloader` into `BrowserFetcher`

Fixes #1748.

test/server/SimpleServer.js

@@ -185,18 +185,20 @@ class SimpleServer {
     if (this._requestSubscribers.has(pathName))
       this._requestSubscribers.get(pathName)[fulfillSymbol].call(null, request);
     const handler = this._routes.get(pathName);
-    if (handler)
+    if (handler) {
       handler.call(null, request, response);
-    else
-      this.defaultHandler(request, response);
+    } else {
+      const pathName = url.parse(request.url).path;
+      this.serveFile(request, response, pathName);
+    }
   }
 
   /**
    * @param {!IncomingMessage} request
    * @param {!ServerResponse} response
+   * @param {string} pathName
    */
-  defaultHandler(request, response) {
-    let pathName = url.parse(request.url).path;
+  serveFile(request, response, pathName) {
     if (pathName === '/')
       pathName = '/index.html';
     const filePath = path.join(this._dirPath, pathName.substring(1));