Add option to support moved filebeat modules

If a module needs to be moved, existing configuration will stop working,
this change adds an option to mark an old module as moved to other name.

This is being considered for the movement of apache2 module to apache to
be coherent with metricbeat module (#9402).

filebeat/fileset/modules.go

@@ -26,6 +26,7 @@ import (
 	"strings"
 
 	"github.com/pkg/errors"
+	yaml "gopkg.in/yaml.v2"
 
 	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/libbeat/kibana"
@@ -57,6 +58,12 @@ func newModuleRegistry(modulesPath string,
 			continue
 		}
 
+		// Look for moved modules
+		if module, moved := getCurrentModuleName(modulesPath, mcfg.Module); moved {
+			logp.Warn("Using old name '%s' for module '%s', please update your configuration", mcfg.Module, module)
+			mcfg.Module = module
+		}
+
 		reg.registry[mcfg.Module] = map[string]*Fileset{}
 		moduleFilesets, err := getModuleFilesets(modulesPath, mcfg.Module)
 		if err != nil {
@@ -180,7 +187,27 @@ func mcfgFromConfig(cfg *common.Config) (*ModuleConfig, error) {
 	return &mcfg, nil
 }
 
+func getCurrentModuleName(modulePath, module string) (string, bool) {
+	moduleConfigPath := filepath.Join(modulePath, module, "module.yml")
+	logp.Info("path: %s, module: %s, path: %s", modulePath, module, moduleConfigPath)
+	d, err := ioutil.ReadFile(moduleConfigPath)
+	if err != nil {
+		return module, false
+	}
+
+	var moduleConfig struct {
+		MovedTo string `yaml:"movedTo"`
+	}
+	err = yaml.Unmarshal(d, &moduleConfig)
+	if err == nil && moduleConfig.MovedTo != "" {
+		return moduleConfig.MovedTo, true
+	}
+
+	return module, false
+}
+
 func getModuleFilesets(modulePath, module string) ([]string, error) {
+	module, _ = getCurrentModuleName(modulePath, module)
 	fileInfos, err := ioutil.ReadDir(filepath.Join(modulePath, module))
 	if err != nil {
 		return []string{}, err