Porting sink change over from 'eventing core'

[matzew]

Fixes #475

Proposed Changes

• porting sink part of fbee61d9eb61a760a49783196e63a4a0c4c789f0 over to contrib, in addtion to @n3wscott's update to the knative.dev/pkg usage

Tested w/ the Kafka Source.

KSVC:

apiVersion: serving.knative.dev/v1beta1
kind: Service
metadata:
  name: k8s-event-display
spec:
  template:
    spec:
      containers:
      - image: docker.io/cds19/event-display

Kafka:

apiVersion: sources.eventing.knative.dev/v1alpha1
kind: KafkaSource
metadata:
  name: kafka-source2
spec:
  consumerGroup: ledemo
  bootstrapServers: my-cluster-kafka-bootstrap.kafka:9092
  topics: my-topic
  sink:
    apiVersion: serving.knative.dev/v1beta1
    kind: Service
    name: k8s-event-display

[matzew]

@bbrowning @lberk I think that's it ...

At least the Kafka Source works w/ the v1beta - does one of you want to test the GH source (no clue on that)

[knative-metrics-robot]

The following is the coverage report on pkg/.
Say /test pull-knative-eventing-contrib-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/controller/sinks/sinks.go	94.7%	95.0%	0.3

[grantr]

/lgtm

Thanks @matzew! Let's get this one in first, then @Harwayne's #476 will add the unit test.

[knative-prow-robot]

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