change providers.json to lock.json

It might not just be for providers, and it's in the plugins dir, so
lock.json seems descriptive enough.

command/init_test.go

@@ -683,7 +683,7 @@ func TestInit_providerLockFile(t *testing.T) {
 	}
 
 	providersLockFile := fmt.Sprintf(
-		".terraform/plugins/%s_%s/providers.json",
+		".terraform/plugins/%s_%s/lock.json",
 		runtime.GOOS, runtime.GOARCH,
 	)
 	buf, err := ioutil.ReadFile(providersLockFile)

command/plugins_lock.go

@@ -11,7 +11,7 @@ import (
 
 func (m *Meta) providerPluginsLock() *pluginSHA256LockFile {
 	return &pluginSHA256LockFile{
-		Filename: filepath.Join(m.pluginDir(), "providers.json"),
+		Filename: filepath.Join(m.pluginDir(), "lock.json"),
 	}
 }
 

command/push_test.go

@@ -125,7 +125,7 @@ func TestPush_goodBackendInit(t *testing.T) {
 		".terraform/",
 		".terraform/plugins/",
 		fmt.Sprintf(".terraform/plugins/%s_%s/", runtime.GOOS, runtime.GOARCH),
-		fmt.Sprintf(".terraform/plugins/%s_%s/providers.json", runtime.GOOS, runtime.GOARCH),
+		fmt.Sprintf(".terraform/plugins/%s_%s/lock.json", runtime.GOOS, runtime.GOARCH),
 		".terraform/terraform.tfstate",
 		".terraform/terraform.tfstate",
 		"main.tf",