lightning: refactor the backend package (#877)

[ti-srebot]

cherry-pick #877 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/902

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/br.git pr/902:release-5.0-2652f252f175

---

What problem does this PR solve?

Split the pkg/lightning/backend package into multiple parts, intended to be the true fix for #837 surviving go mod tidy.

What is changed and how it works?

The backend package itself now only contains the abstract type Backend. The concrete stuff are now split into multiple subpackages:

• Local backend into pkg/lightning/backend/local
• TiDB backend into pkg/lightning/backend/tidb
• Importer backend into pkg/lightning/backend/importer
• KV encoder stuff into pkg/lightning/backend/kv, intended to be merged into pkg/kv in some future PR
• TiKV connection stuff into pkg/lightning/tikv
• The version checking code is split into pkg/lightning/backend/importer, pkg/lightning/backend/local, pkg/lightning/tikvandpkg/version` depending on the component being checked.

Reverted #838 as it is proved to be useless.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• No release note

[lysu]

hi @kennytm please help merge this PR, tidb will introduce dirty dependencies without this PR in release-5.0

[glorv]

/lgtm

[kennytm]

/lgtm

[kennytm]

/merge

[ti-srebot]

/run-all-tests