Configurator: allow querying pkg-config with multiple packages

Desired Behavior

pkg-config supports being queried with multiple packages, but dune-configurator only allows querying with one package at a time. Enabling this support would simplify user code, and save a few invocations.

Example

$ pkg-config --libs --cflags xft x11 wayland-client
-I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/harfbuzz -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/sysprof-6 -pthread -lXft -lX11 -lwayland-client

let from_pkg_config c =
  match C.Pkg_config.get c with
  | None -> ([], [])
  | Some pc ->
    ["xft"; "x11"; "wayland-client"] |>
    List.fold_left (fun ((cflags', libs') as acc) package ->
        match C.Pkg_config.query pc ~package with
        | None -> acc
        | Some { cflags; libs } -> cflags' @ cflags, libs' @ libs) ([], [])
        
(* it's worse imo with *)
val query_expr_err
      :  t
      -> package:string
      -> expr:string
      -> (package_conf, string) result

(* maybe the interface should be *)

val queries : t -> packages:string list -> package_conf option

val queries_exprs_err
  :  t
  -> packages:string list
  -> exprs:string list
  -> (package_conf, string) result
(* or using pairs? *)

Start with dune-configurator's C.Pkg_config.get and query API, then trace how pkg-config invocations are assembled. Compare the proposed multi-package interface options and verify that combined cflags and libs, including error handling, are returned correctly.