Continues standardize of project

[amyangfei]

Is your feature request related to a problem?

The tiflow repo evolves from a monomeric TiCDC repo, to a combination of DM, TiCDC and Dataflow Engine, the evolution history is as follows

• Tracking issue for merging dm and ticdc repos #3194
• Task lists for merging dataflow engine into pingcap/tiflow #5352

Although the migration tasks have finished, but we still have more works to do, including not limited to unify code style, standardize code structure, etc. This issue lists all kinds of such work.

Describe the feature you'd like

• Standardize code structure, ref: Standardized project structure #2199, sub tasks include
  • Move the core code of TiCDC into an independent ticdc dir.
  • Move all public libraries into the github.com/pingcap/tiflow/pkg package.
• Code style
  • Unify import group.
• Engineering implementation
  • replace github.com/pingcap/check with github.com/stretchr/testify in DM. TiCDC has finished this task, ref: Tracking issue for replace github.com/pingcap/check with github.com/stretchr/testify #2164
• Document related
  • refine readme badges. refine readme add release, golang info #5878

Describe alternatives you've considered

No response

Teachability, Documentation, Adoption, Migration Strategy

No response