Running Dash in debugging mode fails during reloading when there's a namespace package present.

Describe your context

dash                      2.0.0
dash-bootstrap-components 0.13.0
dash-core-components      2.0.0
dash-html-components      2.0.0
dash-renderer             1.9.1
dash-table                5.0.0

Describe the bug

Running Dash in debugging mode fails during reloading when there's a namespace package present. I received this traceback:

Traceback (most recent call last):
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "c:\Users\marvinw\.vscode\extensions\ms-python.python-2021.6.944021595\pythonFiles\lib\python\debugpy\__main__.py", line 45, in <module>
    cli.main()
  File "c:\Users\marvinw\.vscode\extensions\ms-python.python-2021.6.944021595\pythonFiles\lib\python\debugpy/..\debugpy\server\cli.py", line 444, in main
    run()
  File "c:\Users\marvinw\.vscode\extensions\ms-python.python-2021.6.944021595\pythonFiles\lib\python\debugpy/..\debugpy\server\cli.py", line 285, in run_file
    runpy.run_path(target_as_str, run_name=compat.force_str("__main__"))
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\runpy.py", line 265, in run_path
    return _run_module_code(code, init_globals, run_name,
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\runpy.py", line 97, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "c:\Users\marvinw\Desktop\Git\sap-explorer\dash\index.py", line 43, in <module>
    app.run_server(debug=True)
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\site-packages\dash\dash.py", line 1958, in run_server
    debug = self.enable_dev_tools(
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\site-packages\dash\dash.py", line 1721, in enable_dev_tools
    component_packages_dist = [
  File "C:\Users\marvinw\.conda\envs\sap-explorer\lib\site-packages\dash\dash.py", line 1726, in <listcomp>
    else package.filename
AttributeError: '_NamespaceLoader' object has no attribute 'filename'

Where I had a namespace package in the same directory. Adding an empty __init__.py to that folder to convert it from a namespace package to a normal package (background info: https://stackoverflow.com/a/48804718/6068036) fixed this error, but imho dash shouldn't block users from also using namespace packages.

Similar issue as in https://github.com/cherrypy/cherrypy/issues/1697.

Expected behavior

Users should be able to use dash, while also having a namespace package in the working directory.

Start in dash/dash.py at enable_dev_tools around lines 1721-1726, where component package filenames are collected. Reproduce debug reloading with a namespace package present and trace the failing loader access. Done means Dash reloads without AttributeError while the namespace package remains usable.