Cluster based on modules

goda graph -cluster now works based on modules instead of nested packages. This gives slightly better structure.

What's Changed

• expr: improve clarity and consistency of help by @ChrisHines in #58
• mod: update go directive and direct dependencies by @ChrisHines in #59
• graph: repository and module clustering by @ChrisHines in #60

Full Changelog: v0.4.5...v0.5.0