Fix Python bootstrap main_rel_path

Use os.path.normpath() to resolve "_main/../repo" to "repo"
and convert forward slashes to backward slashes on Windows.

This fixes an issue where "_main" doesn't exist and in turn
"assert os.path.exists("_main/../repo")" fails. This happens
for example when packaging a py_binary from a foreign repo
into a tar/container.

Author: mering

CHANGELOG.md

@@ -91,6 +91,7 @@ END_UNRELEASED_TEMPLATE
   also retrieved from the URL as opposed to only the `--hash` parameter. Fixes
   [#2363](https://github.com/bazel-contrib/rules_python/issues/2363).
 * (pypi) `whl_library` now infers file names from its `urls` attribute correctly.
+* (py_test, py_binary) Allow external files to be used for main
 
 {#v0-0-0-added}
 ### Added

python/private/python_bootstrap_template.txt

@@ -499,8 +499,7 @@ def Main():
   # The magic string percent-main-percent is replaced with the runfiles-relative
   # filename of the main file of the Python binary in BazelPythonSemantics.java.
   main_rel_path = '%main%'
-  if IsWindows():
-    main_rel_path = main_rel_path.replace('/', os.sep)
+  main_rel_path = os.path.normpath(main_rel_path)
 
   if IsRunningFromZip():
     module_space = CreateModuleSpace()