Reorganize Servo's WPT Python scripts

This change moves all of Servo's WPT Python support scripts into one
directory as they were previously scattered throughout the directory
structure. This should allow more code reuse and make it easier to
understand how everything fits together.

The changes:

- `tests/wpt/update` → `python/wpt/importer`
- `etc/ci/upstream-wpt-changes/wptupstreamer` → `python/wpt/exporter`
- `etc/ci/upstream-wpt-changes/test.py` → `python/wpt/test.py`
- `etc/ci/upstream-wpt-changes/tests` → `python/wpt/tests`
- `tests/wpt/servowpt.py` →
    - `python/wpt/update.py`
    - `python/wpt/run.py`
- `tests/wpt/manifestupdate.py` → `python/wpt/manifestupdate.py`

This change also removes
 - The ability to run the `update-wpt` and `test-wpt` commands without
   using `mach`. These didn't work very well, because it was difficult
   to get all of the wptrunner and mach dependencies installed outside
   of the Python virtualenv. It's simpler if they are always run through
   `mach`.
- The old WPT change upstreaming script that was no longer used.

python/wpt/__init__.py

@@ -0,0 +1,61 @@
+# Copyright 2023 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+import os
+import sys
+
+import mozlog.commandline
+
+SCRIPT_PATH = os.path.abspath(os.path.dirname(__file__))
+SERVO_ROOT = os.path.abspath(os.path.join(SCRIPT_PATH, "..", ".."))
+WPT_PATH = os.path.join(SERVO_ROOT, "tests", "wpt")
+WPT_TOOLS_PATH = os.path.join(WPT_PATH, "web-platform-tests", "tools")
+CERTS_PATH = os.path.join(WPT_TOOLS_PATH, "certs")
+
+sys.path.insert(0, WPT_TOOLS_PATH)
+import localpaths  # noqa: F401,E402
+import wptrunner.wptcommandline  # noqa: E402
+
+
+def create_parser():
+    parser = wptrunner.wptcommandline.create_parser()
+    parser.add_argument('--release', default=False, action="store_true",
+                        help="Run with a release build of servo")
+    parser.add_argument('--rr-chaos', default=False, action="store_true",
+                        help="Run under chaos mode in rr until a failure is captured")
+    parser.add_argument('--pref', default=[], action="append", dest="prefs",
+                        help="Pass preferences to servo")
+    parser.add_argument('--layout-2020', '--with-layout-2020', default=False,
+                        action="store_true", help="Use expected results for the 2020 layout engine")
+    parser.add_argument('--log-servojson', action="append", type=mozlog.commandline.log_file,
+                        help="Servo's JSON logger of unexpected results")
+    parser.add_argument('--always-succeed', default=False, action="store_true",
+                        help="Always yield exit code of zero")
+    parser.add_argument('--no-default-test-types', default=False, action="store_true",
+                        help="Run all of the test types provided by wptrunner or specified explicitly by --test-types")
+    parser.add_argument('--filter-intermittents', default=None, action="store",
+                        help="Filter intermittents against known intermittents "
+                             "and save the filtered output to the given file.")
+    parser.add_argument('--log-raw-unexpected', default=None, action="store",
+                        help="Raw structured log messages for unexpected results."
+                             " '--log-raw' Must also be passed in order to use this.")
+    return parser
+
+
+def update_args_for_layout_2020(kwargs: dict):
+    if kwargs.pop("layout_2020"):
+        kwargs["test_paths"]["/"]["metadata_path"] = os.path.join(
+            WPT_PATH, "metadata-layout-2020"
+        )
+        kwargs["test_paths"]["/_mozilla/"]["metadata_path"] = os.path.join(
+            WPT_PATH, "mozilla", "meta-layout-2020"
+        )
+        kwargs["include_manifest"] = os.path.join(
+            WPT_PATH, "include-layout-2020.ini"
+        )