stack build should check whether git is available

[jwaldmann]

I was running stack build on a (virtual) machine, with some github locations in stack.yaml. I got errors like

 .stack-work/downloaded/Lcy0nQgWISfb/: getDirectoryContents: does not exist (No such file or directory)

The reason is that my machine did not have git installed.

Looks like stack called git, but did not check the exit (error) code.

[sjakobi]

I'm under the impression that there were more problems here:

1. Distro packages of stack ought to have a package dependency on git. Did you install stack through a package manager, @jwaldmann, and what operating system were you running on that VM?
2. stack ought to have failed already when the git clone command failed.

[jwaldmann]

This is on a gitlab builder, usind a docker image for haskell-7.10.3. Here is a typical build that works, but still produces a lot of noise: https://gitlab.imn.htwk-leipzig.de/waldmann/pure-matchbox/builds/931

[Blaisorblade]

We should probably move checkDependency stuff from src/Stack/Setup.hs and use it for this.

[Blaisorblade]

Alternatively, there's isGitInstalled in Stack.PackageIndex, some consolidation might be in order.