bpo-38234: Fix PyConfig_Read() when Py_SetPath() was called (GH-16298)

* If Py_SetPath() has been called, _PyConfig_InitPathConfig() now
  uses its value.
* Py_Initialize() now longer copies path configuration from PyConfig
  to the global path configuration (_Py_path_config).

Misc/NEWS.d/next/C API/2019-09-20-17-22-41.bpo-38234.ZbquVK.rst

+Python ignored path passed to :c:func:`Py_SetPath`, fix Python
+initialization to use the specified path.

Modules/getpath.c

@@ -1213,10 +1213,12 @@ calculate_path_impl(const PyConfig *config,
                 "Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]\n");
     }
 
-    status = calculate_module_search_path(config, calculate,
-                                       prefix, exec_prefix, pathconfig);
-    if (_PyStatus_EXCEPTION(status)) {
-        return status;
+    if (pathconfig->module_search_path == NULL) {
+        status = calculate_module_search_path(config, calculate,
+                                              prefix, exec_prefix, pathconfig);
+        if (_PyStatus_EXCEPTION(status)) {
+            return status;
+        }
     }
 
     status = calculate_reduce_prefix(calculate, prefix, Py_ARRAY_LENGTH(prefix));

PC/getpathp.c

@@ -1003,9 +1003,12 @@ calculate_path_impl(const PyConfig *config,
 
     calculate_home_prefix(calculate, prefix);
 
-    status = calculate_module_search_path(config, calculate, pathconfig, prefix);
-    if (_PyStatus_EXCEPTION(status)) {
-        return status;
+    if (pathconfig->module_search_path == NULL) {
+        status = calculate_module_search_path(config, calculate,
+                                              pathconfig, prefix);
+        if (_PyStatus_EXCEPTION(status)) {
+            return status;
+        }
     }
 
 done:

Python/pathconfig.c

@@ -74,6 +74,13 @@ pathconfig_calculate(_PyPathConfig *pathconfig, const PyConfig *config)
     PyMemAllocatorEx old_alloc;
     _PyMem_SetDefaultAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
+    if (copy_wstr(&new_config.module_search_path,
+                  _Py_path_config.module_search_path) < 0)
+    {
+        status = _PyStatus_NO_MEMORY();
+        goto error;
+    }
+
     /* Calculate program_full_path, prefix, exec_prefix,
        dll_path (Windows), and module_search_path */
     status = _PyPathConfig_Calculate(&new_config, config);