This makes the output more useful for broad searches like on an
entity. Invoices that cross FY boundaries will appear to be paid
without being accrued, and so would appear when we were just
filtering zeroed-out invoices.
If we integrate the aging report into this module in the future,
that'll need to follow different logic, and just filter out
zeroed-out invoices. But the basic balance report and outgoing
report are more workaday tools, where more filtering makes them
more useful.
The old loading strategy didn't load options, which yielded some
spurious errors. It also created awkward duplication of plugin
information in the code as well as the books.
Implement a new loading strategy that works by reading one of the
"main files" under the books/ subdirectory and includes entries
for additional FYs beyond that.
This is still not ideal in a lot of ways. In particular, Beancount can't
cache any results, causing any load to be slower than it theoretically could
be. I expect more commits to follow. But some of them might require
restructuring the books, and that should happen separately.
This module basically reimplements the old payment-report+income-report,
in a single tool (after setting aside some of the checks that have moved
to the plugin). The aging report can be implemented here too when we need
that.
