fix the module path
The module was defined as github.com/ipld/go-codec-dagpb/dagpb, but the repository is at github.com/ipld/go-codec-dagpb. This means that neither can be resolved as a valid Go package import path; the former results in a GitHub 404, and the latter results in a valid GitHub repository which holds a different module. Rod confirmed that the /dagpb suffix wasn't intentional, so just get rid of it. While at it, mod tidy.