Install on Rhino fails

[CoralieMing]

Describe the bug
After following the install instructions on the documentation, the installation of compas_rbe on Rhino fails with the following error AttributeError: module 'compas_rbe' has no attribute '__file__'.

To Reproduce
Steps to reproduce the behavior:

1. Install according to documentation
2. Install on Rhino using: python -m compas_rhino.install -p compas compas_rbe
3. Install fails with the following error:

(workshop) C:\WINDOWS\system32>python -m compas_rhino.install -p compas compas_rbe
Installing COMPAS packages to Rhino 5.0 IronPython lib:
Traceback (most recent call last):
  File "C:\Users\mingc\Anaconda3\envs\workshop\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Users\mingc\Anaconda3\envs\workshop\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\mingc\Anaconda3\envs\workshop\lib\site-packages\compas_rhino\install.py", line 123, in <module>
    install(version=args.version, packages=args.packages)
  File "C:\Users\mingc\Anaconda3\envs\workshop\lib\site-packages\compas_rhino\install.py", line 61, in install
    base_path = _get_package_path(importlib.import_module(package))
  File "C:\Users\mingc\Anaconda3\envs\workshop\lib\site-packages\compas_rhino\install.py", line 21, in _get_package_path
    return os.path.abspath(os.path.join(os.path.dirname(package.__file__), '..'))
AttributeError: module 'compas_rbe' has no attribute '__file__'

Desktop (please complete the following information):

• OS: Windows 10
• Python version 3.6
• Python package manager Anaconda

Additional context
The problem seems to be caused by a PLACEHOLDER file in the source of compas_rbe, after deleting the file locally, the install works correctly.