use InternalValidate on the configschema

This wasn't used until now but we should enforce the schema consistency,
especially when providers start generating their own rather than having
it done by helper/schema.

These will be presented as a warning initially to prevent any
unexpected breakage.

terraform/eval_validate.go

@@ -84,6 +84,34 @@ func (n *EvalValidateProvider) Eval(ctx EvalContext) (interface{}, error) {
 		return nil, diags.NonFatalErr()
 	}
 
+	// Check that the schema is valid, warning the user for now to prevent
+	// breaking existing providers.
+	if err := resp.Provider.Block.InternalValidate(); err != nil {
+		resp.Diagnostics = resp.Diagnostics.Append(tfdiags.Sourceless(
+			tfdiags.Warning,
+			"schema validation error",
+			fmt.Sprintf("The schema for provider %s failed validation with %s.\n\nThis is a bug in the provider, which should be reported in the provider's own issue tracker.\n", n.Addr, err),
+		))
+	}
+	for name, schema := range resp.ResourceTypes {
+		if err := schema.Block.InternalValidate(); err != nil {
+			resp.Diagnostics = resp.Diagnostics.Append(tfdiags.Sourceless(
+				tfdiags.Warning,
+				"schema validation error",
+				fmt.Sprintf("The schema for resource %s in provider %s failed validation with %s.\n\nThis is a bug in the provider, which should be reported in the provider's own issue tracker.\n", name, n.Addr, err),
+			))
+		}
+	}
+	for name, schema := range resp.DataSources {
+		if err := schema.Block.InternalValidate(); err != nil {
+			resp.Diagnostics = resp.Diagnostics.Append(tfdiags.Sourceless(
+				tfdiags.Warning,
+				"schema validation error",
+				fmt.Sprintf("The schema for data source %s in provider %s failed validation with %s.\n\nThis is a bug in the provider, which should be reported in the provider's own issue tracker.\n", name, n.Addr, err),
+			))
+		}
+	}
+
 	configSchema := resp.Provider.Block
 	if configSchema == nil {
 		// Should never happen in real code, but often comes up in tests where