Use SQLite for persistent state

[twpayne]

This is a draft PR. Rough objectives:

• SQLite is more common than BoltDB, so we should use the more standard DB if possible.
• SQLIte, thanks to modernc.org/sqlite, is potentially more portable than BoltDB, and so should help chezmoi run on more platforms (e.g. AIX, build for AIX #2787).
• SQLite supports in-memory databases, so we can replace MockPersistentState with an in-memory SQLite database.

Work in progress...

[twpayne]

Closing this as WONTFIX because, in practice, modernc.org/sqlite doesn't support as many platforms as BoltDB.