Migrate pkg to use the knative.dev/pkg import path

[mattmoor]

I have broken this into two commits:

1. The manual changes I made.
2. The scripted changes I made.

This is a BREAKING change for all contributors

After this has merged, the following changes are needed:

1. Move or recreate your knative/serving directory to ${GOPATH}/src/knative.dev/pkg
2. Run the following sed command pkg any outstanding changes:

sed -i '[email protected]/knative/[email protected]/pkg@g' $(find -name '*.go' | xargs grep github.com/knative/pkg | cut -d':' -f 1 | uniq)

In order to merge this, we are going to have to effectively take a lock on Prow for a few hours because we will need to change the repository configuration and land this change (which also won't pass CI until that change is made).

Related to: knative/serving#971

/hold

Until after we cut 0.7 (even though this is cut for pkg, I don't want to risk the other cuts).

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattmoor

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[chaodaiG]

/retest

[chaodaiG]

/lgtm

[chaodaiG]

/hold cancel

[Harwayne]

1. Move or recreate your knative/serving directory to ${GOPATH}/src/knative.dev/pkg

This should be knative/pkg, right?

[jchesterpivotal]

/lgtm

for same reasons as given in knative/serving#4521.