proxy mirror and mirror API jobs resolved download URLs with a bare
resolver, so npm, cargo, gem, golang, hex, pub, maven and nuget packages
came from the public registries whatever upstream.* says. The resolver
now takes the configured upstreams as base URLs.
npm packages in upstream.npm_routes are fetched from their route and
stored under the route's filename prefix, the way the npm handler does,
so a public package can't stand in for a routed one and clients are
served the mirrored tarball.
Refs #414
