bpo-30876: Relative import from unloaded package now reimports the package (#2639)

instead of failing with SystemError.

Relative import from non-package now fails with ImportError rather than
SystemError.

Python/import.c

@@ -1344,7 +1344,6 @@ resolve_name(PyObject *name, PyObject *globals, int level)
     PyObject *abs_name;
     PyObject *package = NULL;
     PyObject *spec;
-    PyInterpreterState *interp = PyThreadState_GET()->interp;
     Py_ssize_t last_dot;
     PyObject *base;
     int level_up;
@@ -1448,12 +1447,6 @@ resolve_name(PyObject *name, PyObject *globals, int level)
                 "attempted relative import with no known parent package");
         goto error;
     }
-    else if (PyDict_GetItem(interp->modules, package) == NULL) {
-        PyErr_Format(PyExc_SystemError,
-                "Parent module %R not loaded, cannot perform relative "
-                "import", package);
-        goto error;
-    }
 
     for (level_up = 1; level_up < level; level_up += 1) {
         last_dot = PyUnicode_FindChar(package, '.', 0, last_dot, -1);