So far we've been implicitly relying on this by the user passing search
terms that filter out the opening balance transaction. That will stop
happening with the aging report, so we need to do it ourselves.
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.
Building a string and loading it means Beancount can never cache any
load. It only caches top-level file loads because options in the
top-level file can change the semantics of included entries.
Instead use load_file as much as possible, and filter entries as
needed.
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.
