[19.03 backport] Use ocischema package instead of custom handler

[thaJeztah]

backport of moby#40070

Fixes moby#39727
Addresses docker/hub-feedback#1871 (comment)
Relates to distribution/distribution#3024

Previously we were re-using schema2.DeserializedManifest to handle oci
manifests. The issue lies in the fact that distribution started
validating the media type string during json deserialization. This
change broke our usage of that type.

Instead distribution now provides direct support for oci schemas, so use
that instead of our custom handlers.

On 19.03:

$ docker run -it --rm cpuguy83/mobyissue39727:latest cat /foo
Unable to find image 'cpuguy83/mobyissue39727:latest' locally
docker: Error response from daemon: mediaType in manifest should be 'application/vnd.docker.distribution.manifest.v2+json' not 'application/vnd.oci.image.manifest.v1+json'.
See 'docker run --help'.

With this change:

# docker run -it --rm cpuguy83/mobyissue39727:latest cat /foo
Unable to find image 'cpuguy83/mobyissue39727:latest' locally
latest: Pulling from cpuguy83/mobyissue39727
9d48c3bd43c5: Pull complete 
ada774eec8cc: Pull complete 
Digest: sha256:1afcf1bc336b1dbb346fef91b8e2b648f90c801a446cec214c46f544eb3608c8
Status: Downloaded newer image for cpuguy83/mobyissue39727:latest
hello

- Description for the changelog

[thaJeztah]

@cpuguy83

[cpuguy83]

LGTM

[dmcgowan]

LGTM

[thaJeztah]

Some weird failure in docker-py; https://ci.docker.com/public/blue/rest/organizations/jenkins/pipelines/engine/branches/PR-405/runs/2/nodes/203/log/?start=0

[2019-10-17T15:17:38.261Z] docker/api/client.py:261: in _raise_for_status
[2019-10-17T15:17:38.261Z]     response.raise_for_status()
[2019-10-17T15:17:38.261Z] /usr/local/lib/python3.7/site-packages/requests/models.py:940: in raise_for_status
[2019-10-17T15:17:38.261Z]     raise HTTPError(http_error_msg, response=self)
[2019-10-17T15:17:38.261Z] E   requests.exceptions.HTTPError: 404 Client Error: Not Found for url: http+docker://localhost/v1.35/images/hello-world:latest/get

Restarting CI

[andrewhsu]

SGTM