Subscribable types and subtypes v1<>v1alpha1 conversion

[aliok]

Final step of #3474

[mattmoor]

Produced via:
gofmt -s -w $(find -path './vendor' -prune -o -path './third_party' -prune -o -name '*.pb.go' -prune -o -type f -name '*.go' -print)
goimports -w $(find -name '*.go' | grep -v vendor | grep -v third_party | grep -v .pb.go | grep -v wire_gen.go)

[aliok]

Rebased

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/duck/v1alpha1/subscribable_types_conversion.go	91.2%	86.4%	-4.7

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-knative-eventing-upgrade-tests	pull-knative-eventing-upgrade-tests pull-knative-eventing-upgrade-tests pull-knative-eventing-upgrade-tests pull-knative-eventing-upgrade-tests	3/3

Job pull-knative-eventing-upgrade-tests expended all 3 retries without success.

[vaikas]

Thanks for all these!
/lgtm
/approve
/test pull-knative-eventing-upgrade-tests

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aliok, vaikas

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