bpo-42260: Fix _PyConfig_Read() if compute_path_config=0 (GH-23220)

Fix _PyConfig_Read() if compute_path_config=0: use values set by
Py_SetPath(), Py_SetPythonHome() and Py_SetProgramName(). Add
compute_path_config parameter to _PyConfig_InitPathConfig().

The following functions now return NULL if called before
Py_Initialize():

* Py_GetExecPrefix()
* Py_GetPath()
* Py_GetPrefix()
* Py_GetProgramFullPath()
* Py_GetProgramName()
* Py_GetPythonHome()

These functions no longer automatically computes the Python Path
Configuration. Moreover, Py_SetPath() no longer computes
program_full_path.

Python/pylifecycle.c

@@ -1039,7 +1039,7 @@ init_interp_main(PyThreadState *tstate)
     }
 
     // Compute the path configuration
-    status = _PyConfig_InitPathConfig(&interp->config);
+    status = _PyConfig_InitPathConfig(&interp->config, 1);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }