This specialized class is able to carry more context information than
the previous implementation (which reuses ExplicitRequirement). Error
reports can thus provide better messages by introspecting.

This rewrites how a SpecifierRequirement generates candidates, so it

* Always return an AlreadyInstalledCandidate (as long as the version
  satisfies the specifier), even if PackageFinder does not return a
  candidate for the same version.
* Always put the AlreadyInstalledCandidate last, so it's preferred over
  LinkCandidate, preventing version changes if possible.

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.