bpo-45582: Port getpath[p].c to Python (GH-29041)

The getpath.py file is frozen at build time and executed as code over a namespace. It is never imported, nor is it meant to be importable or reusable. However, it should be easier to read, modify, and patch than the previous code.

This commit attempts to preserve every previously tested quirk, but these may be changed in the future to better align platforms.

Modules/posixmodule.c

@@ -4444,6 +4444,33 @@ os__path_splitroot_impl(PyObject *module, path_t *path)
 #endif /* MS_WINDOWS */
 
 
+/*[clinic input]
+os._path_normpath
+
+    path: object
+
+Basic path normalization.
+[clinic start generated code]*/
+
+static PyObject *
+os__path_normpath_impl(PyObject *module, PyObject *path)
+/*[clinic end generated code: output=b94d696d828019da input=5e90c39e12549dc0]*/
+{
+    if (!PyUnicode_Check(path)) {
+        PyErr_Format(PyExc_TypeError, "expected 'str', not '%.200s'",
+            Py_TYPE(path)->tp_name);
+        return NULL;
+    }
+    Py_ssize_t len;
+    wchar_t *buffer = PyUnicode_AsWideCharString(path, &len);
+    if (!buffer) {
+        return NULL;
+    }
+    PyObject *result = PyUnicode_FromWideChar(_Py_normpath(buffer, len), -1);
+    PyMem_Free(buffer);
+    return result;
+}
+
 /*[clinic input]
 os.mkdir
 
@@ -14826,6 +14853,7 @@ static PyMethodDef posix_methods[] = {
     OS__GETFINALPATHNAME_METHODDEF
     OS__GETVOLUMEPATHNAME_METHODDEF
     OS__PATH_SPLITROOT_METHODDEF
+    OS__PATH_NORMPATH_METHODDEF
     OS_GETLOADAVG_METHODDEF
     OS_URANDOM_METHODDEF
     OS_SETRESUID_METHODDEF