Make it easy to run stack on an intranet

This is an enhancement request. The request is to make it easy to run stack on machines that are cut off behind an intranet, via a trusted repository on the intranet side.

The use-case is as follows. Inside a firewall boundary, there are many machines that need stack setups (e.g. build agent servers and personal workstations). There is a policy that such machines can never access the internet, not even via http(s) proxies.

There can be trusted repositories inside the intranet, with curated access to save down selected internet files.

The request is to make it trivial for a stack setup to be told about the trusted repos. Current efforts involve finding and rewriting all urls used throughout the stack setup process. These include github, stackage and hackage.

Instead, have more relative urls, and a short list of top-level sites that stack can access. Then, when wanting to run stack inside an intranet, only a couple of top-level sites have to be renamed in the stack configuration to be intranet sites.

A secondary request would be code to easily set up such mirrors inside an intranet.

Cc: @ndmitchell