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.
Merge pull request #9 from rock-core/fix_importdir_and_git_caching

do not call update_alternates if package.importdir != package.srcdir