Improve pkgsummary reliability and testability

This mostly means consolidating all 'flag' information into a single
structure. Also, arguments from external calls are validated a bit.
test_pkgsummary.py: Initial pkgsummary tests

The primary goal is to check for and flag changes to the external
summary format used by downstream consumers.
conf.py: Add get_std_distro()

Try to determine the right 'standard' distribution for the section,
either 'oldstable', 'stable', 'testing', 'unstable', or 'experimental',
based on the settings from get_distro() and get_distros().
Returns 'unknown' if undetermined.

The tests used for development are included as test-config.py

This adds a "Depends" for python-distro-info to piuparts-common, as a
new component in conf.py.
piuparts-report.py: Default summary dist support

If reporting-sections is 'default', use conf.get_std_distro() to
determine which distribution to assign the results to. If
reporting-sections is 'none', don't make the section summary.