pkg: config reading not supported on Windows

[Alizter]

When building packages on Windows, we get the following error:

PS C:\Users\ali\test> C:\Users\ali\dune\_build\install\default\bin\dune.exe build
Internal error, please report upstream including the contents of _build/log.
Description:
  ("unable to serialize exception",
  { exn = "Failure(\"Windows only. This function isn't implemented.\")" })
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from CamlinternalLazy.force_lazy_block in file "camlinternalLazy.ml",
  line 31, characters 17-27
Re-raised at CamlinternalLazy.force_lazy_block in file "camlinternalLazy.ml",
  line 36, characters 4-11
Called from OpamFilename.prettify_path in file
  "vendor/opam/src/core/opamFilename.ml", line 497, characters 33-54
Called from OpamFile.MakeIO.read_opt in file
  "vendor/opam/src/format/opamFile.ml", line 125, characters 19-42
Called from OpamFile.MakeIO.read in file
  "vendor/opam/src/format/opamFile.ml", line 149, characters 10-20
Called from Dune_rules__Pkg_rules.Install_action.Spec.read_variables in file
  "src/dune_rules/pkg_rules.ml", line 1258, characters 16-45
Called from Dune_rules__Async.async in file "src/dune_rules/async.ml", line
  6, characters 30-36
Called from Dune_rules__Pkg_rules.Install_action.Spec.action in file
  "src/dune_rules/pkg_rules.ml", line 1384, characters 25-75
Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
  line 76, characters 8-11
Re-raised at Stdune__Exn.raise_with_backtrace in file
  "otherlibs/stdune/src/exn.ml", line 38, characters 27-56
Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
  line 76, characters 8-11
Re-raised at Stdune__Exn.raise_with_backtrace in file
  "otherlibs/stdune/src/exn.ml", line 38, characters 27-56
Called from Fiber__Scheduler.exec in file "vendor/fiber/src/scheduler.ml",
  line 76, characters 8-11
-> required by
   ("execute-rule",
   { id = 0
   ; info =
       From_dune_file
         { pos_fname = "<none>"
         ; start = { pos_lnum = 1; pos_bol = 0; pos_cnum = 0 }
         ; stop = { pos_lnum = 1; pos_bol = 0; pos_cnum = 0 }
         }
   })
-> required by ("<unnamed>", ())
-> required by
   ("build-file",
   In_build_dir "_private/default/.pkg/ocaml-system/target/cookie")
-> required by ("pkg-resolve", <opaque>)
-> required by ("pkg-resolve", <opaque>)
-> required by ("pkg-resolve", <opaque>)
-> required by ("native-context", ())
-> required by ("build-contexts", ())
-> required by ("toplevel", ())

It seems that the code from opam that is reading the .config files is a unix-only implementation.

Likely there is something in opamStubs.ml that needs to be supported, but further investigation is needed to work out what.

[Alizter]

We are missing the implementation of shGetFolderPath. I will work on a patch adding it in.