custom execution module directory structure is stripped

Description of Issue

I think it should be mentioned somewhere in https://docs.saltstack.com/en/latest/ref/modules/#writing-execution-modules that directory structured modules like _modules/modname/__init__.py (as opposed to _modules/modname.py) will not get synced by salt-ssh (-thin?) unless they are zipped.

Versions Report

           Salt: 2019.2.1
 
Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.10.1
        libgit2: Not Installed
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.2
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.16 (default, Mar 11 2019, 18:59:25)
   python-gnupg: Not Installed
         PyYAML: 3.13
          PyZMQ: 18.1.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 5.1.1
            ZMQ: 4.3.2
 
System Versions:
           dist:   
         locale: UTF-8
        machine: x86_64
        release: 5.3.4-arch1-1-ARCH
         system: Linux
        version: Not Installed

Start with the linked execution modules documentation page at docs.saltstack.com/en/latest/ref/modules/#writing-execution-modules. Document that directory-structured modules such as _modules/modname/init.py are not synced by salt-ssh unless zipped, and consider the issue complete when this behavior and workaround are clearly described.