check_software: only check shared parts that are used in software

check_software was checking all shared parts, which was an easy
implementation but turned out to be problematic, both when running on
test nodes because if one software installs broken shared parts all
softwares being tested afterwards were failing and also during
development because we needed to prune broken shared parts when
fixing problems reported by check_software.

This inspects buildout's .installed.cfg to guess which shared parts
are used.