Rename Go module to ko.build

[imjasonh]

Splitting out the last remaining item from #791

The project and repo has moved to CNCF ownership, and it's time to update the importpath to reflect that.

• cut v0.14.0, the last real release as github.com/google/ko
• deprecate google/ko and point users to ko.build@v0.15, release that as v0.14.1

// Deprecated: use ko.build instead.
module github.com/google/ko

• rename Go module in go.mod to module ko.build (without the deprecation)
• release v0.15.0 of ko.build, the first release of ko.build
  • Users who go install github.com/google/ko@latest will get v0.14.1 and a notice that it's deprecated in favor of ko.build@v0.15.0
  • dependabot will not automatically upgrade go module consumers across this rename, but hopefully the deprecation warning will help what few code dependents we have.
  • Users can now go install ko.build@latest 🎉
  • Update installation instructions
• update ko-build/setup-ko to install from ko.build@main when version: tip

[imjasonh]

cc @jonjohnsonjr @mattmoor this is the current plan of record, please let me know if you spot any issues. We'll also need to update tf-ko after this change if we want to keep getting ko updates from dependabot.

cc @halvards as Skaffold depends on this module -- does this plan sound right to you?

[mattmoor]

This LGTM

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