books: Let files under books/ include any Beancount files they want.

This provides more flexibility in repository setup while remaining backwards
compatible.

The changes to test_reports_query are just to accommodate the new
transaction in 2020-expenses.beancount.

books.Loader: New loading strategy based on load_file. RT#11034.

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.