Automatic merge from submit-queue.

Restructuring pilot packages

The purpose of this change is:

- consistency with other modules (e.g. mixer)
- better separation of library code from other directories
- more clearly indicating the points of extension
- more clearly indicating those things that are k8s-specific

Overview of the changes:

- Adding `pkg` at the top-level to house all library code.
- Removed `adapter` and moved `config` and `serviceregistry` to the top level under `pkg`
- Moved all service registry implementations under `serviceregistry` (kube, consul, eureka, cloudfoundry)
- Removed `platform` and moved up the remaining `kube` package to the top level under `pkg`
- Moved cmd/pilot-discovery/server to the top-level under `pkg` and renamed to `bootstrap`
- Moved cmd/pilot-discovery/mock to `pkg/proxy/envoy`

The resulting layout for pilot is:

- bin
- cmd
- doc
- docker
- pkg
  - bootstrap
  - config
    - aggregate
    - memory
    - kube
      - crd
        - file
      - ingress
  - dataplane
  - kube
  - model
  - proxy
    - envoy
      - mock
  - serviceregistry
    - aggregate
    - kube
    - consul
    - eureka
    - cloudfoundry
- test
- tools