reduce special error handling to 'replace-provider'

This seeks to further reduce the acceptance of `Error: Invalid legacy
provider address` errors to scenarios where tfmigrate is performing `state
replace-provider`.

This is inspired by code review feedback from @minamijoyo here:
#145 (comment)

tfmigrate/migrator.go

@@ -23,7 +23,7 @@ type Migrator interface {
 
 // setupWorkDir is a common helper function to set up work dir and returns the
 // current state and a switch back function.
-func setupWorkDir(ctx context.Context, tf tfexec.TerraformCLI, workspace string, isBackendTerraformCloud bool, backendConfig []string) (*tfexec.State, func(), error) {
+func setupWorkDir(ctx context.Context, tf tfexec.TerraformCLI, workspace string, isBackendTerraformCloud bool, backendConfig []string, ignoreLegacyStateInitErr bool) (*tfexec.State, func(), error) {
 	// check if terraform command is available.
 	version, err := tf.Version(ctx)
 	if err != nil {
@@ -40,7 +40,7 @@ func setupWorkDir(ctx context.Context, tf tfexec.TerraformCLI, workspace string,
 	log.Printf("[INFO] [migrator@%s] initialize work dir\n", tf.Dir())
 	err = tf.Init(ctx, "-input=false", "-no-color")
 	if err != nil {
-		if supportsStateReplaceProvider && strings.Contains(err.Error(), tfexec.AcceptableLegacyStateInitError) {
+		if supportsStateReplaceProvider && ignoreLegacyStateInitErr && strings.Contains(err.Error(), tfexec.AcceptableLegacyStateInitError) {
 			log.Printf("[INFO] [migrator@%s] ignoring error '%s' initilizing work dir; the error is expected when using Terraform %s with a legacy Terraform state\n", tf.Dir(), tfexec.AcceptableLegacyStateInitError, constraints)
 		} else {
 			return nil, nil, err

tfmigrate/multi_state_migrator.go

@@ -119,14 +119,14 @@ func NewMultiStateMigrator(fromDir string, toDir string, fromWorkspace string, t
 // the Migrator interface between a single and multi state migrator.
 func (m *MultiStateMigrator) plan(ctx context.Context) (*tfexec.State, *tfexec.State, error) {
 	// setup fromDir.
-	fromCurrentState, fromSwitchBackToRemoteFunc, err := setupWorkDir(ctx, m.fromTf, m.fromWorkspace, m.o.IsBackendTerraformCloud, m.o.BackendConfig)
+	fromCurrentState, fromSwitchBackToRemoteFunc, err := setupWorkDir(ctx, m.fromTf, m.fromWorkspace, m.o.IsBackendTerraformCloud, m.o.BackendConfig, false)
 	if err != nil {
 		return nil, nil, err
 	}
 	// switch back it to remote on exit.
 	defer fromSwitchBackToRemoteFunc()
 	// setup toDir.
-	toCurrentState, toSwitchBackToRemoteFunc, err := setupWorkDir(ctx, m.toTf, m.toWorkspace, m.o.IsBackendTerraformCloud, m.o.BackendConfig)
+	toCurrentState, toSwitchBackToRemoteFunc, err := setupWorkDir(ctx, m.toTf, m.toWorkspace, m.o.IsBackendTerraformCloud, m.o.BackendConfig, false)
 	if err != nil {
 		return nil, nil, err
 	}

tfmigrate/state_migrator.go

@@ -104,8 +104,20 @@ func NewStateMigrator(dir string, workspace string, actions []StateAction,
 // We intentionally keep this method private as to not expose internal states and unify
 // the Migrator interface between a single and multi state migrator.
 func (m *StateMigrator) plan(ctx context.Context) (*tfexec.State, error) {
+	ignoreLegacyStateInitErr := false
+	for _, action := range m.actions {
+		// When invoking `state replace-provider`, it's necessary to first
+		// invoke `terraform init`. However, when using a non-legacy Terraform CLI
+		// against a legacy Terraform state, `terraform init` returns an error.
+		// In this scenario, we can (and must) safely ignore the error.
+		if _, ok := action.(*StateReplaceProviderAction); ok {
+			ignoreLegacyStateInitErr = true
+			break
+		}
+	}
+
 	// setup work dir.
-	currentState, switchBackToRemoteFunc, err := setupWorkDir(ctx, m.tf, m.workspace, m.o.IsBackendTerraformCloud, m.o.BackendConfig)
+	currentState, switchBackToRemoteFunc, err := setupWorkDir(ctx, m.tf, m.workspace, m.o.IsBackendTerraformCloud, m.o.BackendConfig, ignoreLegacyStateInitErr)
 	if err != nil {
 		return nil, err
 	}