Collect go vulnerabilities from github api

[sbs2001]

Very recently Github's graphql API started providing for vulnerabilities for go packages. We need to ingest this as part of the github importer

There's also very recent https://github.com/golang/vulndb

[sify21]

Hope this comes soon. Following code shows how golang fetch module versions through a proxy:
https://github.com/golang/go/blob/master/src/cmd/go/internal/modfetch/proxy.go

[sify21]

I made a pull request for this #578