Move the providerSourceAddress fallback to the terraform.withPlugins
function. Since plugins can be arbitrary derivations, there is no
guarantee that this attribute will be exposed.
While doing that I also cleaned the toDrv function to only pass
attributes to the builder which are required by the build.
The Terraform 0.13 fallback slug has changed from
`nixpkgs/<provider-owner>/<provider-name>` to `nixpkgs/<provider-name>`
as the owner is also not always available. As a nixpkgs user, all I know
is that the provider is in nixpkgs and his name, the owner information
is not necessarily easy to get by.
c79ff8ddca