Attestors not registered when using the attestation package directly

[adityasaky]

When I used the https://github.com/in-toto/go-witness/tree/main/attestation package for the attestation collection, I wasn't able to unmarshal a collection (with its collectionattestations) because attestors are registered separately in https://github.com/in-toto/go-witness/blob/main/imports.go. After pulling in these imports (and thus invoking their inits), I was able to proceed.

Note that imports.go is missing material / product, I think they're pulled in somewhere else.

[adityasaky]

FWIW, this probably applies to signers as well, I just didn't run into that.