[BUG] jupyter_mode='external' doesn't work with use_pages=True

[Aleksei-Poliakov]

Describe your context
Please provide us your environment, so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                                     2.18.1
dash_ag_grid                             31.2.0
dash-bootstrap-components                1.6.0
dash-colorscales                         0.0.4
dash-core-components                     2.0.0
dash_cytoscape                           1.0.2
dash_daq                                 0.5.0
dash_html_components                     2.0.0
dash_renderer                            1.9.1
dash-table                               5.0.0
pydash                                   6.0.2

Describe the bug

Using both jupyter_mode='external' and 'use_pages=Truefails on themain` module lookup when ran inside JupyterLab.

File /opt/conda/lib/python3.12/site-packages/dash/dash.py:659, in Dash.init_app(self, app, **kwargs)
    656 self._setup_routes()
    658 _get_app.APP = self
--> 659 self.enable_pages()
    661 self._setup_plotlyjs()

File /opt/conda/lib/python3.12/site-packages/dash/dash.py:2189, in Dash.enable_pages(self)
   2187     return
   2188 if self.pages_folder:
-> 2189     _import_layouts_from_pages(self.config.pages_folder)
   2191 @self.server.before_request
   2192 def router():
   2193     if self._got_first_request["pages"]:

File /opt/conda/lib/python3.12/site-packages/dash/_pages.py:441, in _import_layouts_from_pages(pages_folder)
    438     if "register_page" not in content:
    439         continue
--> 441 module_name = _infer_module_name(page_path)
    442 spec = importlib.util.spec_from_file_location(module_name, page_path)
    443 page_module = importlib.util.module_from_spec(spec)

File /opt/conda/lib/python3.12/site-packages/dash/_pages.py:109, in _infer_module_name(page_path)
    106 parent_module = _path_to_module_name(parent_path)
    108 module_name = f"{parent_module}.{module}"
--> 109 if _module_name_is_package(CONFIG.name):
    110     # Only prefix with CONFIG.name when it's an imported package name
    111     module_name = f"{CONFIG.name}.{module_name}"
    112 return module_name

File /opt/conda/lib/python3.12/site-packages/dash/_pages.py:90, in _module_name_is_package(module_name)
     87 def _module_name_is_package(module_name):
     88     return (
     89         module_name in sys.modules
---> 90         and Path(sys.modules[module_name].__file__).name == "__init__.py"
     91     )

AttributeError: module '__main__' has no attribute '__file__'

Expected behavior

It works similar to how use_pages=True works outside of Jupyter. Not sure how correct the fix is, but I am using this monkey-patch to work around it:

from  dash import _pages
def _hacked_module_name_is_package(module_name):
    return (
            module_name in sys.modules
            and hasattr(sys.modules[module_name], "__file__")
            and Path(sys.modules[module_name].__file__).name == "__init__.py"
    )
_pages._module_name_is_package = _hacked_module_name_is_package