Change 'cosigned' name to 'policy-controller'

[adamd-vmw]

As discussed in sigstore/TSC#9,

cosigned will be renamed policy-controller

[adamd-vmw]

This is to separate the renaming work from the API version bump up and the work to split the controller to its own repo @hectorj2f @coyote240 @elfotografo007 @DennyHoang @vaikas @imjasonh

[imjasonh]

So it sounds like there's around three big refactor-ey changes in the immediate pipeline:

1. bump v1alpha1 -> v1beta1 (done! [cosigned] Bump API version to v1beta1 #1835)
2. rename cosigned -> policy-controller
3. move code out from https://github.com/sigstore/cosign, to https://github.com/sigstore/policy-controller

Do we want (2) to happen before or after (3), or at the same time? Sorry if this has already been discussed and settled.

cc @vaikas

[adamd-vmw]

For our purposes, we actually prefer (2) to happen before (3) simply because when we do (3), we have to do a whole internal approval and build process, which we would prefer to work on after we finish our current product release which needs (2).

but that's just for our own purposes/convenience at vmw