Move the cainjector controller to its own directory

[inteon]

This PR moves the cainjector controller to its own directory since the cainjector is using controller-runtime instead of the framework used by other controllers in /pkg/controller.

Kind

/kind cleanup

Release Note

NONE

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[jetstack-bot]

PR needs rebase.

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[cert-manager-prow]

@inteon: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cert-manager-master-make-verify	207e838	link	true	/test pull-cert-manager-master-make-verify
pull-cert-manager-master-e2e-v1-30-upgrade	207e838	link	true	/test pull-cert-manager-master-e2e-v1-30-upgrade
pull-cert-manager-master-e2e-v1-30	207e838	link	true	/test pull-cert-manager-master-e2e-v1-30

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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