[WIP] Fix/add update gomod

[mhchia]

What was wrong?

#130

How was it fixed?

• Add package.json back, to specify the dependencies on gx.
• Add the python script scripts/update-gomod.py, which parses the dependency on gx, turns gx-hash to git commits, and updates them to go.mod and go.sum using go get.

There are two modes in this script: download, and update.

• download: download and update all dependencies of the current repo according to the gx dependency file package.json, to the path /tmp/gx-git-repos.
• update: parse the dependencies information described by package.json with BFS and get the gx versions in the format in the below for each dependency. Then, search for the git commit or release version with the gx information, in the downloaded repos which are fetched in mode download. Last, use those git commit or release version, we can do GO111MODULE=on go get {repo}@{commit or version} to update the versions to go.mod

{
    "hash": "QmaqGyUhWLsJbVo1QAujSu13mxNjFJ98Kt2VWGSnShGE1Q",
    "name": "go-libp2p-pubsub",
    "version": "0.11.9",
}

Example

$ cd {path/to/sharding-p2p-poc}
$ python scripts/update-gomod.py download  # Download the git repos of all dependencies(including the indirect ones)
$ python scripts/update-gomod.py update
$ GO111MODULE=on go build

Current issues

• Haven't verified whether go get truly update the dependency to go.mod or not
• Seems the version specified in gx(package.json) is not necessarily equaled to the one in git tag

Reference

• gx-gomod: gx-gomod whyrusleeping/gx#200
  • Introduce gomod and proposed a proxy solution for resolution of gx hashes. E.g. version string can be like v2.7.3-gx-ipfs-Qmb3GBFCHMuzmi9EpH3pxpYBiviyc3tEPyDQHxZJQJSxj9 in gx-gomod whyrusleeping/gx#200 (comment)
• Gx hash mapped to git commit?: Link a gx version/hash with a git hash whyrusleeping/gx#190
  • Seems currently it is not supported
• Reconsidering gx: Reconsidering GX ipfs/kubo#5850
  • discussion about gomod v.s. gx
• Make GX pleasant to use: Making GX pleasant to use whyrusleeping/gx#179 (comment)
  • I haven't fully understand the proposed changes, great discussion tho
• gxed organization: https://github.com/gxed
  • The gx'ed repos are put here.

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[mhchia]

Note: I'd like to close this PR and reopen it after confirming all deps can be resolved correctly. Moved the script update-gomod.py to another repo https://github.com/mhchia/py-gx-gomod