Look up variables even if they refer to other packages

1. Fixes the problem of not resolving variables that refer to other
   packages
2. Silences the warning about unresolved variables if the variable
   refers to a package that is not installed.
3. Keeps the warning if the variable is not referring to a package and
   undefined OR the package is installed and is missing the definition.

Closes ocaml-opam#45.

lib/switch_context.ml

@@ -17,11 +17,35 @@ let user_restrictions t name =
   OpamPackage.Name.Map.find_opt name t.constraints
 
 let env t pkg v =
-  if List.mem v OpamPackageVar.predefined_depends_variables then None
-  else (
-    let r = OpamPackageVar.resolve_switch ~package:pkg t.st v in
-    if r = None then OpamConsole.warning "Unknown variable %S" (OpamVariable.Full.to_string v);
-    r
+  match List.mem v OpamPackageVar.predefined_depends_variables with
+  | true -> None
+  | false -> (
+    match OpamPackageVar.resolve_switch ~package:pkg t.st v with
+    | Some _ as resolved -> resolved
+    | None -> (
+      match OpamVariable.Full.package v with
+      | None -> (
+        (* a local variable that is unknown *)
+        OpamConsole.warning "Unknown variable %S" (OpamVariable.Full.to_string v);
+        None)
+      | Some pkg_name ->
+        let installed_package = OpamPackage.Set.find_opt (fun package ->
+            OpamPackage.Name.equal (OpamPackage.name package) pkg_name)
+          t.st.installed
+        in
+        match installed_package with
+        | None ->
+          (* [v] refers to a package that is not installed in the switch *)
+          None
+        | Some package -> (
+          let unscoped = OpamVariable.Full.(self (variable v)) in
+          match OpamPackageVar.resolve_switch ~package t.st unscoped with
+          | None ->
+            OpamConsole.warning "Unknown variable %S" (OpamVariable.Full.to_string v);
+            None
+          | Some _ as resolved -> resolved
+        )
+      )
   )
 
 let filter_deps t pkg f =