Fix #361

%{package.section:variable}% is confusing but '.' is the only non-ambiguous package name separator in OPAM currently. So instead, warm the user when he tries to use that syntax incorrectly.
Revert "[tests] provide more checking"

This reverts commit ae17b348c2b4c711aa105e08ff839e5650896ac2 because 01754cf1e28619e378c80ceb1feb959a145e075e correctly warms the user about his mistakes now.