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.
