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.

lib/helper.js

@@ -13,10 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+const fs = require('fs');
+const path = require('path');
 
 const debugError = require('debug')(`puppeteer:error`);
 /** @type {?Map<string, boolean>} */
 let apiCoverage = null;
+let projectRoot = null;
 class Helper {
   /**
    * @param {Function|string} fun
@@ -41,6 +44,17 @@ class Helper {
     }
   }
 
+  /**
+   * @return {string}
+   */
+  static projectRoot() {
+    if (!projectRoot) {
+      // Project root will be different for node6-transpiled code.
+      projectRoot = fs.existsSync(path.join(__dirname, '..', 'package.json')) ? path.join(__dirname, '..') : path.join(__dirname, '..', '..');
+    }
+    return projectRoot;
+  }
+
   /**
    * @param {!Object} exceptionDetails
    * @return {string}