packagesdb - Refactor out get_rdep_dict() from calc_rrdep_counts().

Also, cache the dictionary for performance.
packagesdb.py - Use get_package() in get_rdep_dict() to resolve virtual pkgs.
packagesdb - Add routine to calc rdep metrics by package as needed.
packagesdb - Add rdep metric accessors at the PackagesDB level.
Remove need for chdir() before loading packagesdb.

... in piuparts-report and detect_well_known_errors.

Establish the full path to the directory as 'prefix'.
The calc_rrdep_counts() call is hiding the potential need for the
directory to be correct in other places. Removing the rrdep calc
would otherwise cause a crash.
Use the on-demand versions of the rdep metric calls.
packagesdb.py - Remove the Package-level interface for rdep metrics.
piuparts-report.py - Avoid sorting passed files by block count.

There was still a case where a full accounting of rdep metrics was
called for - the sort of successfully-tested in piuparts-report.
Avoid the sort call to block_count if we know it is not going to
affect the sort.

With this change, piuparts-reports now runs at 35 seconds per section.
piuparts-report.py - Refactor with_count in write_state_pages().
Changelog entry for rdep metric rework.
piupartslib/packagesdb.py - Add rdep chain length metric calc.

Provide a means for detecting packages which support a long chain
of reverse dependencies, so that testing priorities can concentrate
on whittling them down.
piupartslib/packages.db - Prioritize by rdep chain len in reserve()

Use the rdep_chain_len metric to proportionally prioritize packages
returned by reserve() according to the length of the longest reverse
dependency chain length.