Remove warnings reported by xref

Replace rebar_util:get_cwd/0 by rebar_utils:get_cwd/0.  Luckily the
misspelt module name doesn't seem to have caused any harm, since
rebar_app_utils:is_app_dir/0 and rebar_rel_utils:is_rel_dir/0 aren't
called (only their /1 counterparts).

src/rebar_app_utils.erl

@@ -42,7 +42,7 @@
 %% ===================================================================
 
 is_app_dir() ->
-    is_app_dir(rebar_util:get_cwd()).
+    is_app_dir(rebar_utils:get_cwd()).
 
 is_app_dir(Dir) ->
     AppSrc = filename:join(Dir, "src/*.app.src"),

src/rebar_rel_utils.erl

@@ -29,7 +29,7 @@
 -export([is_rel_dir/0, is_rel_dir/1]).
 
 is_rel_dir() ->
-    is_rel_dir(rebar_util:get_cwd()).
+    is_rel_dir(rebar_utils:get_cwd()).
 
 is_rel_dir(Dir) ->
     Fname = filename:join([Dir, "reltool.config"]),