fix: normalize argv0 so runfiles root can be found on windows with bazel 9 (#2481)

When the shell test invokes the python binary, it uses a combination of
forward slashes and
backslashes. Under Bazel 9, that mixture of slashes is preserved. This
later breaks a regex
that looks for the OS-specific path separator.

To fix, normalize forward slashes to the OS path separator.

Oddly, it's not Bazel that is passing the mixture of slashes (it's the
shell), but behavior seems to
vary based on which version of Bazel is used.

Along the way, copy the nicer `print_verbose` function from the stage2
bootstrap into the old
bootstrap. It prints debug information in a nicer format.

Work towards https://github.com/bazelbuild/rules_python/issues/2469

python/private/python_bootstrap_template.txt

diff --git a/python/private/python_bootstrap_template.txt b/python/private/python_bootstrap_template.txt
index 0f9c90b..e3b39e3 100644
--- a/python/private/python_bootstrap_template.txt
+++ b/python/private/python_bootstrap_template.txt
@@ -89,9 +89,28 @@
   """Finds the real Python binary if it's not a normal absolute path."""
   return FindBinary(module_space, PYTHON_BINARY)
 
-def PrintVerbose(*args):
-  if os.environ.get("RULES_PYTHON_BOOTSTRAP_VERBOSE"):
-    print("bootstrap:", *args, file=sys.stderr, flush=True)
+def print_verbose(*args, mapping=None, values=None):
+    if os.environ.get("RULES_PYTHON_BOOTSTRAP_VERBOSE"):
+        if mapping is not None:
+            for key, value in sorted((mapping or {}).items()):
+                print(
+                    "bootstrap:",
+                    *args,
+                    f"{key}={value!r}",
+                    file=sys.stderr,
+                    flush=True,
+                )
+        elif values is not None:
+            for i, v in enumerate(values):
+                print(
+                    "bootstrap:",
+                    *args,
+                    f"[{i}] {v!r}",
+                    file=sys.stderr,
+                    flush=True,
+                )
+        else:
+            print("bootstrap:", *args, file=sys.stderr, flush=True)
 
 def PrintVerboseCoverage(*args):
   """Print output if VERBOSE_COVERAGE is non-empty in the environment."""
@@ -157,6 +176,12 @@
     return runfiles_dir
 
   stub_filename = sys.argv[0]
+  # On Windows, the path may contain both forward and backslashes.
+  # Normalize to the OS separator because the regex used later assumes
+  # the OS-specific separator.
+  if IsWindows:
+    stub_filename = stub_filename.replace("/", os.sep)
+
   if not os.path.isabs(stub_filename):
     stub_filename = os.path.join(os.getcwd(), stub_filename)
 
@@ -380,9 +405,9 @@
   # type: (str, str, list[str], dict[str, str]) -> ...
   """Executes the given Python file using the various environment settings."""
   os.environ.update(env)
-  PrintVerbose("RunExecv: environ:", os.environ)
+  print_verbose("RunExecv: environ:", mapping=os.environ)
   argv = [python_program, main_filename] + args
-  PrintVerbose("RunExecv: argv:", python_program, argv)
+  print_verbose("RunExecv: argv:", python_program, argv)
   os.execv(python_program, argv)
 
 def _RunForCoverage(python_program, main_filename, args, env,
@@ -453,6 +478,10 @@
   return ret_code
 
 def Main():
+  print_verbose("initial argv:", values=sys.argv)
+  print_verbose("initial cwd:", os.getcwd())
+  print_verbose("initial environ:", mapping=os.environ)
+  print_verbose("initial sys.path:", values=sys.path)
   args = sys.argv[1:]
 
   new_env = {}