builder: merge/refactor census_summary modules

Follow-on to #1440 The builder has two different source files named `census_summary.py`. One computes the stored `census["census_info"]["summary"]` table and the other computes slightly different statistics shown in the builder's log/report we store elsewhere (not user-facing). Having two source files with the same filename is confusing in itself, and there is also the risk/history that they quantify things in slightly different ways that can lead to confusion when folks are comparing the table with the logs.