Change Prelude/ directory to be a submodule

[Gabriella439]

... as suggested by @f-f

Now that there is a separate git repository for the Prelude, we can use it
as a submodule instead of mirroring the files between the two repositories

[Gabriella439]

Hmmm. It looks like the use of submodules causes evaluation errors under the current Nix logic:

command `nix-prefetch-git /var/lib/hydra/scm/git/12a6923f1f6a30e5b8cd6617716519885ff93d2bb618d20552eb1e3d5de8a621 03d56ed227ee199bb8fcb257985164193e2cedda' failed with exit status 256 at /nix/store/rak0ad5jqs1jy20kaq8zws22044blh6d-hydra-2017-11-21/libexec/hydra/lib/Hydra/Helper/Nix.pm line 428.

I might have to postpone this until I figure out how to fix that problem

[ocharles]

We use sub modules just fine. Are you sure the submodule commit you're referring to actually exists on the remote?

[Gabriella439]

@ocharles: Is that separate from pushing the branch?

[ocharles]

Nope, it looks like everything is correct here, as I can follow the commit via GitHub's UI. I would follow journalctl -u hydra-evaluator -u hydra-queue-runner -f. IIRC, there's a log message that shows up there that doesn't show up in the web UI. Should shine some light on why nix-prefetch-git is failing.

[Gabriella439]

Closing this in favor of making the Prelude a remote import as suggested in dhall-lang/dhall-lang#240 (comment)