Add golang.org/x/exp/apidiff as a linter

[sfllaw]

Your feature request related to a problem? Please describe.

When a developer makes a backwards-incompatible change to their API, it would be nice for a linter to warn them so they could make a conscious decision to do so.

Describe the solution you'd like.

Add golang.org/x/exp/apidiff as a disabled by default linter.

When enabled, golangci-lint should:

1. Check if the version in go.mod is v1 or greater. If the module is v0, then backwards-incompatible changes are OK and the linter shouldn’t be run. Although it might make sense to add an option to force-v0: true.
2. Use GOPROXY to pull the same major version of the current module, which should be the latest public version. If this major version doesn’t exist, then this must be a major version bump, so backwards-incompatible changes are OK and the linter shouldn’t be run.
3. Run apidiff against the public version of the module and warn

Describe alternatives you've considered.

There doesn’t seem to be a public alternative to apidiff, as of 2021-10-28.

Additional context.

Unfortunately, apidiff does not use the analysis framework.

It also isn’t aware of GOPROXY, so there probably needs to be a shim that reads the go.mod file and downloads the appropriate public version using golang.org/x/tools/go/packages. Thankfully, apidiff.Changes takes *types.Package arguments, so this shim shouldn’t be too bad.

I recommend apidiff be disabled by default since it is still experimental. That being said, Kubernetes is running it as part of their CI pipeline: https://github.com/kubernetes/utils/actions/runs/77631466/workflow.

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