Replace node-fetch with the Node.js https module

[lydell]

The uses of node-fetch were so few and trivial that I took a shot at getting rid of one more dependency!

Notes:

• node-fetch automatically sets these headers by default:

  accept: */*
  accept-encoding: gzip,deflate
  host: <the host in question>
  user-agent: node-fetch/1.0 (+https://github.com/bitinn/node-fetch)

  While https.get by default only sets host. Tested using https://hookbin.com/.

  We make requests to https://elm-package-cache-psi.vercel.app, and it does not seem to care about accept and user-agent. When sending accept-encoding, it does not respond with a content-encoding.

  So we don’t need to set any headers.

• node-fetch automatically follows redirects, while https.get does not. We’re not encountering any redirects though.

• The old code using node-fetch did not check for status codes. The new code does. So if the server is down you now get a nicer error than a JSON parse syntax error.