Humble suggestion / shameless plug: try using Sourcegraph to read through some of the excellent codebases mentioned in other posts. This is basically our prime use case (I'm one of the creators): reading through and understanding code structure with jump-to-def / find-refs / symbol search in your browser. I'd suggest starting with the main function and tracing through the static structure of the codebase from there:

https://sourcegraph.com/github.com/golang/go/-/blob/src/net/... https://sourcegraph.com/github.com/mholt/caddy@master/-/blob... https://sourcegraph.com/github.com/btcsuite/btcd@master/-/bl... https://sourcegraph.com/github.com/camlistore/camlistore@mas... https://sourcegraph.com/github.com/boltdb/bolt@master/-/blob... https://sourcegraph.com/github.com/go-kit/kit/-/blob/example...