chore: implement persistent terraform directories (experimental) (#20563)

Prior to this, every workspace build ran `terraform init` in a fresh
directory. This would mean the `modules` are downloaded fresh. If the
module is not pinned, subsequent workspace builds would have different
modules.

provisioner/terraform/executor.go

@@ -41,7 +41,7 @@ type executor struct {
 	// cachePath and files must not be used by multiple processes at once.
 	cachePath     string
 	cliConfigPath string
-	files         tfpath.Layout
+	files         tfpath.Layouter
 	// used to capture execution times at various stages
 	timings *timingAggregator
 }
@@ -536,7 +536,11 @@ func (e *executor) graph(ctx, killCtx context.Context) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	args := []string{"graph"}
+	args := []string{
+		"graph",
+		// TODO: When the plan is present, we should probably use it?
+		// "-plan=" + e.files.PlanFilePath(),
+	}
 	if ver.GreaterThanOrEqual(version170) {
 		args = append(args, "-type=plan")
 	}

provisioner/terraform/modules.go

@@ -58,7 +58,7 @@ func parseModulesFile(filePath string) ([]*proto.Module, error) {
 // getModules returns the modules from the modules file if it exists.
 // It returns nil if the file does not exist.
 // Modules become available after terraform init.
-func getModules(files tfpath.Layout) ([]*proto.Module, error) {
+func getModules(files tfpath.Layouter) ([]*proto.Module, error) {
 	filePath := files.ModulesFilePath()
 	if _, err := os.Stat(filePath); os.IsNotExist(err) {
 		return nil, nil

provisioner/terraform/serve.go

@@ -161,7 +161,7 @@ func (s *server) startTrace(ctx context.Context, name string, opts ...trace.Span
 	))...)
 }
 
-func (s *server) executor(files tfpath.Layout, stage database.ProvisionerJobTimingStage) *executor {
+func (s *server) executor(files tfpath.Layouter, stage database.ProvisionerJobTimingStage) *executor {
 	return &executor{
 		server:        s,
 		mut:           s.execMut,