WIP/Search upwards for package directory

[BardurArantsson]

FOR REVIEW ONLY

[BardurArantsson]

High-level overview of what I've done here:

• Added a new SandboxMetadata that contains both the sandbox
  directory and the config file path.
• I've changed the UseSandbox construct to take such a
  SandboxMetadata value -- this simplifies the lookup logic
  drastically since it can be unified in a single function which
  handles both the lookup of the .cabal file and the
  configuration file. There are some cases error cases that would
  be very hard to detect if we didn't combine the logic this.
  (See getSandboxMetadata for the search/lookup logic.)

Observation: The config flag handling seems rather messy -- I
find it very hard to judge which configuration is actually being
used.

[BardurArantsson]

See also comments on the #2810 issue regarding implementation

[BardurArantsson]

Oh, yeah, I haven't spent any time actually testing this yet -- I just want to get feedback on the approach before I spend any time on adding/updating tests. I expect a feature this complex will require some PackageTests at the very least.

[hvr]

Thanks for doing this.

Could you maybe outline the algorithm you use (or plan to use) for locating the various config-files?

[BardurArantsson]

I'm closing this for now. I think it'll need a bit of reworking and most of all TESTS. Hopefully, #2797 will be enough :).

[BardurArantsson]

Meh, meant #2868. (Hard to follow all the indirections!)