Resolve gx dependency for go mod

[mhchia]

What is wrong?

Though we have changed to go mod after #121, we still need to enforce the dependency relationship handled in gx, due to the fact that go-libp2p is still using gx to handle the dependencies.

How can it be fixed?

Have a tool to resolve gx dependencies, by going through the package.json file, mapping the dependencies' ipfs hashes to the git commit hashes, and recording them in go.sum