Support remote (git) repos

[judah]

Support specifying extra-deps as github repos, like stack does.

Use the new stack-1.6 syntax.

[philderbeast]

In the meantime stack2cabal can be used to work around this. For example I was getting this error initially;

> stack exec pier -- build
Unpacking GHC
Installing GHC locally
Making global DB relative
Error when running Shake build system:
* Library flight-route
Couldn't find package "hcoord-utm"
CallStack (from HasCallStack):
  error, called at src/Pier/Build/Config.hs:136:19 in
pier-0.1.0.0-BenaSEYnr8Kf85ig9KgUP:Pier.Build.Config

Here's the git section of stack.yaml for the flare-timing project;

- location:
    git: https://github.com/blockscope/hcoord.git
    commit: b2fb75029f0374e48caf4cbbfcfcc2fd674cb4f3 
  subdirs:
  - hcoord
  - hcoord-utm
  extra-dep: true
- location:
    git: https://github.com/haskell/fgl.git
    commit: e29503775e474b7a7cd8951b6bebcf1529b888b5
  extra-dep: true
- location:
    git: https://github.com/brunjlar/stack2cabal
    commit: a8a4cdcacd697cee648af93856735f15de5987a6
  extra-dep: true
- location:
    git: https://github.com/judah/pier
    commit: 65f526e242cae3f2255ead9fbaaa13cf85f9022a
  extra-dep: true

To make the workaround, bring down git dependencies locally with;

stack exec stack2cabal -- .

Then reference those packages in pier.yaml;

packages:
- .stack2cabal/hcoord/hcoord
- .stack2cabal/hcoord/hcoord-utm
- .stack2cabal/fgl
...

The build now completes;

> stack exec pier -- build
...
Build completed in 2:07m