command/init: Restore the unconstrained provider warnings
command/init: Restore the unconstrained provider warnings

When a provider dependency is implicit rather than explicit, or otherwise
when version constraints are lacking, we produce a warning recommending
the addition of explicit version constraints in the configuration.

This restores the warning functionality from previous Terraform versions,
adapting it slightly to account for the new provider FQN syntax and to
recommend using a required_providers block rather than version constraints
in "provider" blocks, because the latter is no longer recommended in the
documentation.

rnaveiras/terraform,heimweh/terraform,rnaveiras/terraform,tommynsong/terraform,hashicorp/terraform,ewbankkit/terraform,VladRassokhin/terraform,apparentlymart/terraform,tommynsong/terraform,ewbankkit/terraform,ewbankkit/terraform,pdecat/terraform,VladRassokhin/terraform,pdecat/terraform,tommynsong/terraform,pdecat/terraform,rnaveiras/terraform,pdecat/terraform,heimweh/terraform,apparentlymart/terraform,heimweh/terraform,pryorda/terraform,hashicorp/terraform,ewbankkit/terraform,pryorda/terraform,apparentlymart/terraform,apparentlymart/terraform,VladRassokhin/terraform,pryorda/terraform,apparentlymart/terraform,hashicorp/terraform,VladRassokhin/terraform,heimweh/terraform,heimweh/terraform