NPM package doesn't include contracts build artifacts

[tmigone]

trafficstars

As reported by @fordN

Looks like the hardhat.config has them now configured to go to /build/abis and build/types, but the node package config doesn't include that dir

   "dist/**/*",
   "README.md",
   "LICENSE",
   "addresses.json"
 ],

[abarmat]

The prepublish script moves some of the artifacts (ABI and types) to the NPM package before we publish it. We never shipped the source code or anything else -> https://github.com/graphprotocol/contracts/blob/dev/scripts/prepublish