Move PD to github.com/tikv/pd

[nolouch]

We are going to move PD to TiKV GitHub organization.
Our motivation is TiKV has a strong dependency on PD, they together form a distributed KV system.

Filing an issue to sort through what needs to be done:

• Announce to TiDB/TiKV communities. (11 AM, UTC+8, August 17, 2020)
• Transfer ownership
• Update all internal github.com/pingcap/pd import paths in all branches
• Set up new CIs, Bots in New Org
• Reconfigure automated testing: Travis, Jenkins, Circle...
• Update the import paths and go.mod of all downstream dependent projects in pingcap

Some relative issues:

• Can all the released TiDB versions still compile successfully?
  Yes. github will redirect all requests to the new URL and go mod download still works.
• Consider canceling the semantic Import mechanism in PD
  Because we meet some problems like: Will cause cherry-pick to conflict with the lower version branch of a release, Weird dependency problems like pd and pd/v4 appeared at the same time in one repo, Need to resolve lots of dependency problem after we bump to a higher version

[HunDunDM]

If we want to use vanity imports, we may need https://github.com/GoogleCloudPlatform/govanityurls.

[disksing]

Can we close it now?

[nolouch]

yes