Stackage.CLI library

[chrisdone]

Make a small library in the .cabal stackage-cli package with a module Stackage.CLI which exports a wrapper to optparse-applicative or similar with a preconfigured way of listing a summary (--summary probably) line of help.

Then the main executable can import this module to determine the correct prefix (--summary probably) to pass to the plugin commands to get a line for its --help display.

[chrisdone]

This library can also expose the means to find plugins (via a Text string) and execute them, returning a status code at the end. Then the main stackage binary (#3) has little work to do. This will be handy for plugins that call other plugins. To be clear: Given the dynamic nature of whether plugins are available or not, it should be treated as a Text lookup, not as a static import of a library.

[chrisdone]

Remaining items:

• [x] We need to pass-through the arguments of a command straight to the command, and not be interpreted by the stackage executable.
• [ ] Running commands should allow stdin/stdout interaction.

[chrisdone]

@DanBurton I fixed item one in https://github.com/fpco/stackage-cli/commit/e91fcf275f88c93eeb2ea11da22730c522d22302 and 220ae05. I think you're good on the second point.

[DanBurton]

Running commands via Stackage.CLI.runStackagePlugin allows stdin/stdout interaction. In the repo's current state, I can build and correctly interact with the stackage purge command.

[DanBurton]

Should be ready to close.

[chrisdone]

Confirmed in purge