database: Fetch and index fork and stars for GitHub and BitBucket.

Store and show if a package is a fork, how many imports and stars
(followers in BitBucket) for each package in the search result list.
Currently we have over 90% packages coming from these two VCS's.
Corresponding UI change is made to show these additional information
under each package's import path.

Change-Id: I669755d4b905f360918d38e8600534a61a449ba4
Reviewed-on: https://go-review.googlesource.com/24173
Reviewed-by: Alan Donovan <adonovan@google.com>
9 files changed