Fix incremental issue with namespace packages

Fixes python#12664

See python#18907 which is another possible fix

I'm not sure why this logic is actually needed given that we should
record the dependency from import analysis in fastparse... maybe just
because it adjusts priority?

I can't write a good test for this because it requires something in site_packages, but here's a minimal repro:
```
set -eux
rm -rf repro
mkdir repro
cd repro

SITEPACK=env/site-packages
mkdir -p $SITEPACK

mkdir $SITEPACK/ruamel
mkdir $SITEPACK/ruamel/yaml

printf 'from ruamel.yaml.main import *' > $SITEPACK/ruamel/yaml/__init__.py
printf 'import ruamel.yaml' > $SITEPACK/ruamel/yaml/main.py
printf '' > $SITEPACK/ruamel/yaml/py.typed

printf 'import ruamel.yaml' > a.py
printf 'import a' > main.py

rm -rf .mypy_cache
PYTHONPATH=$SITEPACK mypy main.py
PYTHONPATH=$SITEPACK mypy main.py
```

Author: hauntsaninja

mypy/semanal.py

@@ -404,9 +404,6 @@ class SemanticAnalyzer(
     cur_mod_id = ""  # Current module id (or None) (phase 2)
     _is_stub_file = False  # Are we analyzing a stub file?
     _is_typeshed_stub_file = False  # Are we analyzing a typeshed stub file?
-    imports: set[str]  # Imported modules (during phase 2 analysis)
-    # Note: some imports (and therefore dependencies) might
-    # not be found in phase 1, for example due to * imports.
     errors: Errors  # Keeps track of generated errors
     plugin: Plugin  # Mypy plugin for special casing of library features
     statement: Statement | None = None  # Statement/definition being analyzed
@@ -445,7 +442,6 @@ def __init__(
         self.saved_locals: dict[
             FuncItem | GeneratorExpr | DictionaryComprehension, SymbolTable
         ] = {}
-        self.imports = set()
         self._type = None
         self.type_stack = []
         # Are the namespaces of classes being processed complete?
@@ -3114,9 +3110,6 @@ def visit_import_all(self, i: ImportAll) -> None:
                 # if '__all__' exists, all nodes not included have had module_public set to
                 # False, and we can skip checking '_' because it's been explicitly included.
                 if node.module_public and (not name.startswith("_") or "__all__" in m.names):
-                    if isinstance(node.node, MypyFile):
-                        # Star import of submodule from a package, add it as a dependency.
-                        self.imports.add(node.node.fullname)
                     # `from x import *` always reexports symbols
                     self.add_imported_symbol(
                         name, node, context=i, module_public=True, module_hidden=False

mypy/semanal_main.py

@@ -405,11 +405,6 @@ def semantic_analyze_target(
         )
         if isinstance(node, Decorator):
             infer_decorator_signature_if_simple(node, analyzer)
-    for dep in analyzer.imports:
-        state.add_dependency(dep)
-        priority = mypy.build.PRI_LOW
-        if priority <= state.priorities.get(dep, priority):
-            state.priorities[dep] = priority
 
     # Clear out some stale data to avoid memory leaks and astmerge
     # validity check confusion