Add integration tests for various go mod corner cases.

[Cynocracy]

"Fixes" #171 (where I was confused by the behavior of modules) by ensuring ko behaves similarly to go build in various edge cases.

[jonjohnsonjr]

I'm not sure if we'll always want to keep this exact behavior, but having a test is nice to know if we change it 😄 lgtm once you fix the build failures

[Cynocracy]

Oh, because it's not actually obvious: I wrote the test cases before checking behavior :) So at least I'm convinced this behavior is correct.

[jonjohnsonjr]

Same error :/

I think you need a build tag that doesn't get used so that it doesn't try to build it, we did something similar in go-containerregistry: https://github.com/google/go-containerregistry/pull/599/files#diff-39bad9c50a0f30bd6c067b83431bec76R1

[Cynocracy]

Ah, I missed that I didn't have the tools build tag, yeah that's definitely necessary.

[Cynocracy]

Any suggestions on what to set for KO_DOCKER_REPO ?

[jonjohnsonjr]

Any suggestions on what to set for KO_DOCKER_REPO ?

I think the current integration tests use the --local flag to just write it into the docker daemon, which doesn't require KO_DOCKER_REPO to be set, if you wanna do that.

[Cynocracy]

Ah, perfect, that works for me, thank you :)

[Cynocracy]

Ready! I think :)

[jonjohnsonjr]

Thanks for doing this, I don't think I would have had to patience :)