The candidate creation logic is further moved into the factory. The
factory would use pkg_resources.get_distribution() to find a matching
distribution for a givan InstallationCandidate. If found, the Candidate
would be created based on that found distribution, instead of the link.

--ignore-installed is implemented as to always use the link to create
candidates, even if an installed distribution is found.

Make assert_installed actually check the provided entries against the
pip --list output.

Add assert_not_installed to check for the reverse.