allow empty message in .warn and .error, the way .message does already
do not call update_alternates if package.importdir != package.srcdir

importdir != srcdir really says that the current package is not the
one being imported, and that the alternates list cannot be generated,
so skip the update.

This fixes a bunch of warnings when using importdir with cache, where
the alternates would basically be modified for each sub-package.
release 1.8.1
1.8.1 was already released, this is 1.8.2