warn when submodules missing

[Geometer1729]

Here I just steal the code from mina to give a useful warning when using nix develop .
closes #1896

[Fizzixnerd]

on Ubuntu with nix installed, I tried:

git checkout brian/submodule-warn
nix develop .

The output was:

fizzixnerd@o1-walkerbook:~/src/o1js$ nix develop .
warning: Git tree '/home/fizzixnerd/src/o1js' is dirty
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:34:12:
           33|
           34|   strict = derivationStrict drvAttrs;
             |            ^
           35|

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/g8zzlf6drg73c987ii390yicq4c0j778-source/pkgs/stdenv/generic/make-derivation.nix:348:7

       … while evaluating attribute 'nativeBuildInputs' of derivation 'nix-shell'
         at /nix/store/g8zzlf6drg73c987ii390yicq4c0j778-source/pkgs/stdenv/generic/make-derivation.nix:392:7:
          391|       depsBuildBuild              = elemAt (elemAt dependencies 0) 0;
          392|       nativeBuildInputs           = elemAt (elemAt dependencies 0) 1;
             |       ^
          393|       depsBuildTarget             = elemAt (elemAt dependencies 0) 2;

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: lib.fileset.unions: Element 0 (/nix/store/llhbnzfyq8jsy5qwdpi65j9birn2bvgh-source/src/bindings/ocaml/dune) is a path that does not exist.
           To create a file set from a path that may not exist, use `lib.fileset.maybeMissing`.

[Fizzixnerd]

Rerunning pin.sh after checkout fixed it.

[Fizzixnerd]

Tried a fresh clone, and it works.