Really fix the weird uninstall problem

I've finally nailed down issue ocaml#348 (and ocaml#362). The issue was indeed caused by da0d7de:
* This commit is implementing an useful optimization: in case the remove command is 'ocamlfind remove XXX' only, then OPAM don't create ~/.opam/<switch>/build/<pkg> anymore. Which means increased uninstall time.
* However, the uninstall command was still run in ~/.opam/<switch>/build/<pkg>. If that directory does not exists, the command was just dropped silently.
* OPAM automatically removes ~/.opam/<switch>/lib/<pkg>, so when the package was not using C bindings (with stub files in ~/.opam/<switch>/lib/stublibs) the bug was hidden
* Using OPAM with OPAMKEEPBUILDIR=1 (which means that the build dir wasn't removed during upgrades) hides the bug

So now I've fixed the bug:
* when we do an exec in a given dir, OPAM fails if the dir doesn't exist
* on uninstall, if the build dir is not there run the uninstall command at the OPAM root

This should fix ocaml#362

src/client/opamSolution.ml

@@ -256,14 +256,18 @@ let proceed_to_delete ~rm_build t nv =
       && OpamState.mem_repository t nv
       && not (List.for_all use_ocamlfind remove) then (
         try extract_package t nv
-        with _ -> OpamFilename.mkdir p_build;
+        with _ -> ()
       );
+      let exec_dir =
+        if OpamFilename.exists_dir p_build
+        then p_build
+        else t.root in
       try
         OpamGlobals.msg "%s\n" (string_of_commands remove);
         OpamFilename.exec
           ~add_to_env:env.add_to_env
           ~add_to_path:[OpamPath.Switch.bin t.root t.switch]
-          p_build
+          exec_dir
           remove
       with _ ->
         ();

src/core/opamFilename.ml

@@ -61,7 +61,7 @@ let in_dir dirname fn =
     OpamGlobals.error_and_exit "%s does not exist!" dirname
 
 let exec dirname ?(add_to_env=[]) ?(add_to_path=[]) cmds =
-  OpamSystem.in_dir (Dir.to_string dirname)
+  in_dir dirname
     (fun () ->
       OpamSystem.commands
         ~add_to_env