✨ Return an error for duplicate generators

[rainest]

Return an error from controller-gen if the command includes multiple instances of the same generator. For example:

$ controller-gen "+crd:generateEmbeddedObjectMeta=true" rbac:roleName=manager-role crd webhook paths="~/src/operator/apis" +output:crd:artifacts:config=/tmp/wat

Error: multiple instances of 'crd' generator specified

I hit the issue described in #671 and was quite confused by it.

I'm not sure if you should ever be able to specify multiple of the same generator in a single run, as this causes apparently unintended effects. In my case, only honoring the output directive for one and dumping the other in the default location, with no obvious way to apply the output configuration to both.

I'm also unsure if there's actually a way to provide a warning versus returning an error, so this does the latter. I'm not that familiar with the codebase but didn't find anything that looked like warning-level logging. Prints to stderr only show up in the top-level command code, and it doesn't make sense to issue them directly from the generator library.

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[varshaprasad96]

/lgtm

Seems reasonable to me, though I'm doubtful if we need to error out or just split out a warning. But I don't see a reason in mentioning multiple generators in the cli.

[vincepri]

/approve
/ok-to-test

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rainest, varshaprasad96, vincepri

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[sbueringer]

/cherry-pick release-0.12

[k8s-infra-cherrypick-robot]

@sbueringer: new pull request created: #835

In response to this:

/cherry-pick release-0.12

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