integrate the gopath hacks to allow for codegen anywhere

[n3wscott]

Proposed Changes

• Use GOPATH hacks to codegen anywhere.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: n3wscott

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[codecov]

Codecov Report

Merging #4621 (85c7f09) into master (e5eee31) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4621   +/-   ##
=======================================
  Coverage   81.31%   81.31%           
=======================================
  Files         290      290           
  Lines        8176     8176           
=======================================
  Hits         6648     6648           
  Misses       1128     1128           
  Partials      400      400           

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[zhongduo]

/lgtm
The complexity is in hack/codegen-library.sh instead of this PR. The fact that all our CI checks run this update-codegen.sh script make me believe it is well tested, as long as all the related PRs pass the tests. (Note that sample-controller does not, but this might be sth else as you said in slack.
/hold
in case you want a second opinion.

[n3wscott]

Sample controller now does this: knative-extensions/sample-controller#371 😄

/unhold