Issue legacy path warnings to stderr

To make it possible to ignore the warning e.g. when running yadm
introspect repo in the bash completion.

test/test_unit_issue_legacy_path_warning.py

@@ -34,8 +34,8 @@ def test_legacy_warning(tmpdir, runner, yadm, upgrade, override, legacy_path):
     """
     run = runner(command=['bash'], inp=script)
     assert run.success
-    assert run.err == ''
+    assert run.out == ''
     if legacy_path and (not upgrade) and (not override):
-        assert 'Legacy paths have been detected' in run.out
+        assert 'Legacy paths have been detected' in run.err
     else:
-        assert 'Legacy paths have been detected' not in run.out
+        assert 'Legacy paths have been detected' not in run.err

yadm

@@ -1506,7 +1506,7 @@ function issue_legacy_path_warning() {
     path_list="$path_list    * $legacy_path"$'\n'
   done
 
-  cat <<EOF
+  cat <<EOF >&2
 
 **WARNING**
   Legacy paths have been detected.