Remove anyhow from dirmgr.

This is a large diff.

The unwrap() additions to the test cases are there to avoid having to add error variants for AuthorityBuilderError, DirMgrConfigBuilderError, DownloadScheduleConfigBuilderError, NetworkConfigBuilderError and FallbackDirBuilderError. While make check passes without this change, make test does not.

Maybe those error should be exported as well from src/err.rs, individually or grouped under ConfigBuilderError?

If unwraps are ok in tests, then the unused_wraps clippy override could maybe be eliminated by not returning Result<> in those four test functions.