Replace github.com/hpcng/golang-x-crypto fork by github.com/ProtonMail/crypto fork

[cclerget]

The fork github.com/hpcng/golang-x-crypto wasn't updated since a long time and could be hard to maintain and is rather sensible. The fork github.com/ProtonMail/crypto provides the missing feature Singularity requires for private key encryption and is pretty much up to date. Additionally one of developers of github.com/ProtonMail/crypto is currently the maintainer of golang.org/x/crypto package, so it seems a better option.

NOTE: require to replace SerializePrivate call by SerializePrivateWithoutSign if the private key is already encrypted

[cclerget]

Looks like this is not currently possible without slightly break SIF package, not a huge change but github.com/ProtonMail/crypto has a different function prototype for openpgp.CheckDetachedSignature forcing SIF package to also use github.com/ProtonMail/crypto with a replace statement in go.mod which may not be desirable as it would force project importing SIF package to use the replace statement and could be a mess if a project also rely on golang.org/x/crypto package