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.
Ultimately this is going to be a tool that can generate nicely-formatted
spreadsheets from arbitrary bean-queries. This initial version doesn't
generate spreadsheets yet, but it does integrate our usual books-loading
tools and rewrite rules into existing bean-query functionality, so it's a
start. It also has some of the query building and parsing that higher-level
spreadsheets will need.