Merge source across required_providers blocks

[alisdair]

If multiple terraform.required_providers blocks are in the config, we already merge the version constraints for each provider. We should also merge the source attribute, and warn if there are duplicates.

Consider the following configuration:

terraform {
  required_providers {
    foo = {
      version = "1.0.0"
    }
  }
}

terraform {
  required_providers {
    foo = {
      source = "abc/foo"
    }
  }
}

Before this commit, this would result in a provider requirement for "foo" with version "1.0.0", but no source. This commit merges the source attribute from the second block into this requirement.

[justincampbell]

Also FWIW, I don't believe the Terraform Registry codebase uses any Markdown output from this library.

[alisdair]

After some more discussion about how this would be used in Terraform, we're not going ahead with this change.