This is for the same reasons as ParseJson, that is so that we can limit
the scope of the proto namespace differences between internal and
external.

This reverts commit d88e15ce.

This reverts commit 48d833a9, reversing
changes made to 66e9d8e8.

As opposed to a flat directory, we now have the following structure:

- security
  -context
  - credentials
    - composite
    - fake
    - google_default
    - iam
    - jwt
    - oauth2
    - plugin
    - ssl
  - transport
  - util

We have not refactored the test code yet but this PR is already large
enough...

much metadata and refuses to send responses with too much metadata.

For some definition of "smart"... client_channel simply passes along
pollset/pollset_set, removing the need to instantiate a pollset_set in
the subchannel_call_holder: it's now up to the LB policies to handle the
pollset/pollset_set.