configs/configupgrade: Upgrade provider addresses

Both resource blocks and module blocks contain references to providers
that are expressed as short-form provider addresses ("aws.foo" rather than
"provider.aws.foo").

These rules call for those to be unwrapped as naked identifiers during
upgrade, rather than appearing as quoted strings. This also introduces
some further rules for other simpler meta-arguments that are required
for the test fixtures for this feature.

configs/configupgrade/test-fixtures/valid/provider-addrs/input/provider-addrs.tf

@@ -0,0 +1,16 @@
+provider "test" {
+  alias = "baz"
+}
+
+resource "test_instance" "foo" {
+  provider = "test.baz"
+}
+
+module "bar" {
+  source = "./baz"
+
+  providers = {
+    test     = "test.baz"
+    test.foo = "test"
+  }
+}

configs/configupgrade/test-fixtures/valid/provider-addrs/want/provider-addrs.tf

@@ -0,0 +1,16 @@
+provider "test" {
+  alias = "baz"
+}
+
+resource "test_instance" "foo" {
+  provider = test.baz
+}
+
+module "bar" {
+  source = "./baz"
+
+  providers = {
+    test     = test.baz
+    test.foo = test
+  }
+}

configs/configupgrade/test-fixtures/valid/provider-addrs/want/versions.tf

@@ -0,0 +1,3 @@
+terraform {
+  required_version = ">= 0.12"
+}

configs/configupgrade/upgrade_native.go

@@ -210,6 +210,34 @@ func (u *Upgrader) upgradeNativeSyntaxFile(filename string, src []byte, an *anal
 			// the special lifecycle arguments below.
 			rules := justAttributesBodyRules(filename, body, an)
 			rules["source"] = noInterpAttributeRule(filename, cty.String, an)
+			rules["version"] = noInterpAttributeRule(filename, cty.String, an)
+			rules["providers"] = func(buf *bytes.Buffer, blockAddr string, item *hcl1ast.ObjectItem) tfdiags.Diagnostics {
+				var diags tfdiags.Diagnostics
+				subBody, ok := item.Val.(*hcl1ast.ObjectType)
+				if !ok {
+					diags = diags.Append(&hcl2.Diagnostic{
+						Severity: hcl2.DiagError,
+						Summary:  "Invalid providers argument",
+						Detail:   `The "providers" argument must be a map from provider addresses in the child module to corresponding provider addresses in this module.`,
+						Subject:  &declRange,
+					})
+					return diags
+				}
+
+				// We're gonna cheat here and use justAttributesBodyRules to
+				// find all the attribute names but then just rewrite them all
+				// to be our specialized traversal-style mapping instead.
+				subRules := justAttributesBodyRules(filename, subBody, an)
+				for k := range subRules {
+					subRules[k] = maybeBareTraversalAttributeRule(filename, an)
+				}
+				buf.WriteString("providers = {\n")
+				bodyDiags := upgradeBlockBody(filename, blockAddr, buf, subBody.List.Items, body.Rbrace, subRules, adhocComments)
+				diags = diags.Append(bodyDiags)
+				buf.WriteString("}\n")
+
+				return diags
+			}
 
 			printComments(&buf, item.LeadComment)
 			printBlockOpen(&buf, blockType, labels, item.LineComment)
@@ -298,6 +326,7 @@ func (u *Upgrader) upgradeNativeSyntaxResource(filename string, buf *bytes.Buffe
 
 	rules := schemaDefaultBodyRules(filename, schema, an, adhocComments)
 	rules["count"] = normalAttributeRule(filename, cty.Number, an)
+	rules["provider"] = maybeBareTraversalAttributeRule(filename, an)
 
 	printComments(buf, item.LeadComment)
 	printBlockOpen(buf, blockType, labels, item.LineComment)
@@ -321,6 +350,8 @@ func (u *Upgrader) upgradeNativeSyntaxProvider(filename string, buf *bytes.Buffe
 	}
 	schema := providerSchema.Provider
 	rules := schemaDefaultBodyRules(filename, schema, an, adhocComments)
+	rules["alias"] = noInterpAttributeRule(filename, cty.String, an)
+	rules["version"] = noInterpAttributeRule(filename, cty.String, an)
 
 	printComments(buf, item.LeadComment)
 	printBlockOpen(buf, "provider", []string{typeName}, item.LineComment)