The tsearch2-driver package has code that checks if tsearch2.sql is already installed and if not, tries to find it and load it upon install. Its not fool proof but it helps.
Gustaf is correct, since we support at least 2 versions of PostgreSQL and tsearch2 is often updated or backported independent of PostgreSQL it doesn't make sense to try to keep it in the OpenACS tree. You need to install tsearch2 anyway to get tsearch2.so so if you want the tsearch2-driver package to work.
