…sqlglot)

This is the minimal set of fixes needed for `separate=True` to build and run
correctly against sqlglot, a ~100-module project with cross-group class
inheritance, generator helper classes, non-ext subclasses with fast methods,
and mutually-dependent compiled modules. Each of the fixes below is a real
bug that was never hit by mypy itself (mypy's setup.py uses multi_file on
Windows only, never separate=True) or by the toy fixtures in mypyc's
TestRunSeparate.

1. Non-extension classes never have vtables -- short-circuit is_method_final
   to True for them so codegen doesn't try to index into a vtable that
   compute_vtable skipped.

2. emit_method_call: under separate=True, a method's FuncIR body may live in
   another group while only its FuncDecl is visible here. Use method_decl(name)
   instead of get_method(name).decl -- the decl is enough to emit a direct C
   call. Split native_function_type to accept a decl too.

3. Cross-group native/Python-wrapper calls weren't routing through the
   exports-table indirection at a dozen sites in emitwrapper / emitfunc /
   emitclass. Added Emitter.native_function_call(decl) and
   Emitter.wrapper_function_call(decl) helpers and migrated all offending
   sites. Also made CPyPy_* wrapper declarations needs_export=True so those
   symbols reach the exports table.

4. Defer cross-group imports to shim load time. The shared lib's exec_
   function used to PyImport_ImportModule sibling groups at PyInit time,
   which re-enters the enclosing package's __init__.py mid-flight and blows
   up on partial-init attribute walks. Split exec_ into a self-contained
   capsule-setup phase (runs in PyInit) and a deferred ensure_deps_<short>()
   (runs from the shim just before per-module init). Shim uses
   PyImport_ImportModuleLevel with a non-empty fromlist so the lookup
   returns the leaf directly via sys.modules, and fetches capsules via
   PyObject_GetAttrString instead of PyCapsule_Import (which itself performs
   the same dotted attribute walk).

5. Fix broken fallback in lib-rt CPyImport_ImportFrom: the code tried
   PyObject_GetItem(module, fullname) where it intended PyImport_GetModule
   (comment says as much). Modules don't implement __getitem__, so the
   fallback always raised TypeError. Also Py_XDECREF the potentially-NULL
   package_path in the error path.

6. Incremental-mode plumbing for separate=True: compile_modules_to_ir now
   syncs freshly built ClassIR/FuncIR into deser_ctx so later cache-loaded
   SCCs can resolve cross-SCC references. load_type_map tolerates mypy's
   synthetic TypeInfo entries (e.g. "<subclass of X and Y>") that have no
   corresponding mypyc ClassIR.

Also adds three regression tests targeted to fail on TestRunSeparate
without the fixes above:

- testSeparateCrossGroupEnumMethod exercises fix #1.
- testSeparateCrossGroupGenerator exercises fix #2.
- testSeparateCrossGroupInheritedInit exercises fix #3.