Introspect downloaded modules/packages/things for go.mod files

[vatine]

At the moment, there's no stats on "is a thing go-module enabled or not", it would probably be interesting to see how that evolves over time. It should be relatively simple to locate the downloaded source from the name and just do a file probe to see if there's a go.mod file (and/or investigate if we can convince either go list or go mod do that for us).