bpo-35134: Add Include/cpython/import.h header file (GH-14213)

* Add Include/cpython/import.h and Include/internal/pycore_import.h
  header files.
* Move _PyImport_ReInitLock() to the internal C API. Don't export the
  symbol anymore.
diff --git a/Include/cpython/import.h b/Include/cpython/import.h
new file mode 100644
index 0000000..8dd7a0c
--- /dev/null
+++ b/Include/cpython/import.h
@@ -0,0 +1,57 @@
+#ifndef Py_CPYTHON_IMPORT_H
+#  error "this header file must not be included directly"
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+PyMODINIT_FUNC PyInit__imp(void);
+
+PyAPI_FUNC(int) _PyImport_IsInitialized(PyInterpreterState *);
+
+PyAPI_FUNC(PyObject *) _PyImport_GetModuleId(struct _Py_Identifier *name);
+PyAPI_FUNC(PyObject *) _PyImport_AddModuleObject(PyObject *name,
+                                                 PyObject *modules);
+PyAPI_FUNC(int) _PyImport_SetModule(PyObject *name, PyObject *module);
+PyAPI_FUNC(int) _PyImport_SetModuleString(const char *name, PyObject* module);
+
+PyAPI_FUNC(void) _PyImport_AcquireLock(void);
+PyAPI_FUNC(int) _PyImport_ReleaseLock(void);
+
+PyAPI_FUNC(PyObject *) _PyImport_FindBuiltin(
+    const char *name,            /* UTF-8 encoded string */
+    PyObject *modules
+    );
+PyAPI_FUNC(PyObject *) _PyImport_FindExtensionObject(PyObject *, PyObject *);
+PyAPI_FUNC(PyObject *) _PyImport_FindExtensionObjectEx(PyObject *, PyObject *,
+                                                       PyObject *);
+PyAPI_FUNC(int) _PyImport_FixupBuiltin(
+    PyObject *mod,
+    const char *name,            /* UTF-8 encoded string */
+    PyObject *modules
+    );
+PyAPI_FUNC(int) _PyImport_FixupExtensionObject(PyObject*, PyObject *,
+                                               PyObject *, PyObject *);
+
+struct _inittab {
+    const char *name;           /* ASCII encoded string */
+    PyObject* (*initfunc)(void);
+};
+PyAPI_DATA(struct _inittab *) PyImport_Inittab;
+PyAPI_FUNC(int) PyImport_ExtendInittab(struct _inittab *newtab);
+
+struct _frozen {
+    const char *name;                 /* ASCII encoded string */
+    const unsigned char *code;
+    int size;
+};
+
+/* Embedding apps may change this pointer to point to their favorite
+   collection of frozen modules: */
+
+PyAPI_DATA(const struct _frozen *) PyImport_FrozenModules;
+
+#ifdef __cplusplus
+}
+#endif