reorganize pd related code under pd package

[nirrozenbaum]

as a follow up to #179, two comments from the code review should be addressed:

• all pd related code should be under pd package. we may have this structure for example:
  pkd/pd
  pkd/pd/plugins
  pkd/pd/scheduling

• create pd consts and reuse the same consts across the code. additionally, make the consts publicly accessible so other libraries like the sidecar can access the same consts.

cc: @elevran

[elevran]

thanks @nirrozenbaum
Not sure if we need the directory structure under pd or just use file names in the same directory (which may help in keeping somethings private).

[nirrozenbaum]

closing as not relevant. after config api was introduced the existing code changes significantly.