Replace generate_imports python script with mage imports

[ChrsMark]

This PR tries to refactor and unify the way we create include/list.go files. This came after we moved light modules to OSS and while trying to create a light module under OSS. In that case this module will be included in list.go file despite the fact that it has no .go files. This was not a problem when light modules where under x-pack because there the list.go was created by mage and the script was taking into consideration the existence of .go files in a module so as to decide if it should be included in list.go.

So far we had:

• x-pack way: https://github.com/elastic/beats/blob/master/x-pack/metricbeat/magefile.go#L128
• oss way: https://github.com/elastic/beats/blob/master/metricbeat/Makefile#L46

This PR refactors the OSS part so as to make use of the mage respective target likewise x-pack.

cc: @jsoriano , @exekias

[jsoriano]

Yet another python script in the way to be removed 😃

[ChrsMark]

This is sth which requires a careful look, not sure if it's ok to remove. Maybe @fearful-symmetry can comment here.

[fearful-symmetry]

The new go generator has its own code for linking that, so I'm not sure what's using this?

beats/generator/common/beatgen/setup/setup.go

Lines 55 to 62 in 72f2bf9

	func LinkImportsHelper() error {
	vendorPath := "./vendor/github.com/"
	pwd, err := os.Getwd()
	if err != nil {
	return errors.Wrap(err, "error gettting current directory")
	}
	return sh.Run("ln", "-sf", filepath.Join(pwd, vendorPath, "elastic/beats/metricbeat/scripts/generate_imports_helper.py"), filepath.Join(pwd, vendorPath, "elastic/beats/script/generate_imports_helper.py"))
	}