dmypy's `--use-fine-grained-cache` crashes with `KeyError` on namespace packages

This is possibly related to #10735.

To Reproduce & Traceback

Create a minimal project as follows:

project/
  mypy.ini
      [mypy]
      files = src/
  src/
    pkg/
      __init__.py
      nspkg/
        mod.py
            a: int = ""

Reproduce with these steps:

1. Navigate to project/

2. $ mypy --cache-fine-grained
   src/pkg/nspkg/mod.py:1: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
   

3. $ dmypy run -- --use-fine-grained-cache
   Daemon started
   Success: no issues found in 2 source files
   

   (I don't think this should say Success given the errors from Step 2., but that's a separate issue.)
4. Change mod.py, e.g. modify the sole line to a: int = "a"
5. Run the daemon twice:

   • $ dmypy run -- --use-fine-grained-cache
     src/pkg/nspkg/mod.py:1: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
     

   • $ dmypy run -- --use-fine-grained-cache
     Daemon crashed!
     Traceback (most recent call last):
       File ".../mypy/dmypy_server.py", line 237, in serve
         ...
       File ".../mypy/dmypy_server.py", line 768, in find_reachable_changed_modules
      worklist.append(BuildSource(graph[dep].path, graph[dep].id, followed=True))
                                  ~~~~~^^^^^
     KeyError: 'pkg.nspkg'
     

Your Environment

• Mypy version used: 1.19.1
• Python version used: Python 3.13

Reproduziere den Namespace-Package-Fall mit dmypy und --use-fine-grained-cache und untersuche anschließend dmypy_server.py im Umfeld von find_reachable_changed_modules und Zeile 768, wo der Traceback in graph nach pkg.nspkg sucht. Verwende die aufgeführten Befehle, um zu überprüfen, dass der Daemon beim zweiten Lauf nicht mehr abstürzt und weiterhin den Zuweisungsfehler meldet.