Improvements to stubgenc (#14564)

This bundles together a handful of improvements to stubgenc. Each one
has its own commit, but it can be somewhat time consuming to get a PR
reviewed and merged, so I'm putting them together for the sake of
expediency. I'll break them up if it is preferred.

An overview of the main changes:

- infer return types for known special methods: previously only argument
types were inferred
- check class docstrings for signatures in addition to `__init__`:
shiboken binding generator is known to put constructor signatures in
class docstrings rather than on `__init__`
- use the list of analyzed modules to produce better imports: previously
when given `foo.Bar.Spangle`, it was assumed that `import foo.Bar`
should be added, however, it could be that `Bar.Spangle` refers to
nested classes within the module `foo`.
- when fixing up types, also process children of compound types
- evaluate descriptors when getting members: necessary for shiboken
bindings

mypy/stubgen.py

@@ -1720,6 +1720,7 @@ def generate_stubs(options: Options) -> None:
             )
 
     # Separately analyse C modules using different logic.
+    all_modules = sorted(m.module for m in (py_modules + c_modules))
     for mod in c_modules:
         if any(py_mod.module.startswith(mod.module + ".") for py_mod in py_modules + c_modules):
             target = mod.module.replace(".", "/") + "/__init__.pyi"
@@ -1728,7 +1729,9 @@ def generate_stubs(options: Options) -> None:
         target = os.path.join(options.output_dir, target)
         files.append(target)
         with generate_guarded(mod.module, target, options.ignore_errors, options.verbose):
-            generate_stub_for_c_module(mod.module, target, sig_generators=sig_generators)
+            generate_stub_for_c_module(
+                mod.module, target, known_modules=all_modules, sig_generators=sig_generators
+            )
     num_modules = len(py_modules) + len(c_modules)
     if not options.quiet and num_modules > 0:
         print("Processed %d modules" % num_modules)