I found some module duplications which may be imported earlier without
detailed comparison with go.mod. Which makes the project kind of
messy. Specifically,
We use github.com/cespare/xxhash/2 in go code but its elder version
in related tests.
Both github.com/peterbourgon/ff/v3 and its elder version are used to
construct cmdline tools.
This commit merges the two duplicated imports above and refactors some
necessary code.
