mod: Merge duplicated module imports

Created by: blanet

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.

Test plan

CI

restored source branch github/fork/blanet/xx/merge-duplicate-imports