feat: go modules support (#2584)

Adds support for updating go modules (found in go.mod) and the accompanying go.sum checksum files.

Limitations:

- github.com modules only (but supports "redirects" to github e.g. by gopkg.in and golang.org)
- skips major upgrades > v1
- no vendoring support

Closes #933, Closes #2578, Closes #2579