Add support for go modules

[andrew]

⚠️ work in progress ⚠️

Adds support for acting as a go modules proxy server like https://proxy.golang.org, caching the zips to ipfs along the way.

Also lots of refactoring to make the codebase less npm specific, still some more work to do:

• go server needs to cache metadata as well as zips
• forest packages only lists npm packages atm
• forest republish only reads package-lock.json files, should also read go.sum files, output should be a combination of both
• forest seed may not work for go modules at the moment
• verification of modules annoucned on pubsub needs to be done using sum.golang.org server
• forest verify command only checks npm packages atm
• forest update doesn't check go modules right now
• forest preload only looks for package-lock.json files, should also search for go.sum files
• go server currently runs on a separate port, needs to be combined with server.js
• need to watch https://index.golang.org/index for new updates to go modules in the background
• forest watch only watches for new npm releases
• go support needs documenting
• flag for some cli commands to only operate on one manager, i.e. --manager npm
• config/unconfig commanda only changes npm config (go modules enabled using GOPROXY= env var)

[andrew]

Note: requests to proxy.golang.org must be lowercase, otherwise it errors: golang/go#40202 (comment)

[andrew]

Go's integrity checking code for zips: https://github.com/golang/mod/blob/master/sumdb/dirhash/hash.go (h1 is base64 encoded sha256), need to implement the HashZip and Hash1 functions from that file.

https://www.npmjs.com/package/node-stream-zip should be helpful

[andrew]

Merged to avoid more conflicts in the near future