This commit fixes a go 1.15 vet check.
In doing so it uncovers that the time caveat check is actually reversed.
Since we should check that the caveat is added, we should only fail the
check when the caveat prefix is not equal.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
