port 2P step 1: vendor CPython 2.7.18 as the mtpython static library

The embedded 40250 script layer needs its own interpreter on five platforms.
2.7 is end-of-life, so nothing can be fetched from the target SDKs and the
source is vendored (official 2.7.18 tarball, trimmed to 25MB; Lib/ stays for
step 3's python27.zip).

cpython-2.7.18/CMakeLists.txt builds one `mtpython` static library from exactly
the 133 objects the reference libpython2.7.a contains, off by default behind
-DMTGODOT_EMBED_PYTHON=ON. The source list and the built-in module table
(config/config.c, 39 entries) are shared; pyconfig.h is a probe result and is
not, so each platform keeps its own under config/<platform>/, regenerated by
tools/py_embed/gen_pyconfig.sh and checked against the shared table.

Three vendor patches, documented in docs/THIRD-PARTY.md: configure/configure.ac
learn arm64 on macOS, and posixmodule.c undefines the process-control calls it
hard-defines past pyconfig.h when building for iOS.

macOS (ctest python.embed: every builtin imports, codecs and pickle work off the
vendored Lib/), Android arm64 and iOS arm64 build. Linux needs a Linux host and
Windows needs a MinGW-vs-MSVC decision; both are noted for step 4.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/third_party/cpython-2.7.18/Lib/importlib/__init__.py

@@ -0,0 +1,38 @@
+"""Backport of importlib.import_module from 3.x."""
+# While not critical (and in no way guaranteed!), it would be nice to keep this
+# code compatible with Python 2.3.
+import sys
+
+def _resolve_name(name, package, level):
+    """Return the absolute name of the module to be imported."""
+    if not hasattr(package, 'rindex'):
+        raise ValueError("'package' not set to a string")
+    dot = len(package)
+    for x in xrange(level, 1, -1):
+        try:
+            dot = package.rindex('.', 0, dot)
+        except ValueError:
+            raise ValueError("attempted relative import beyond top-level "
+                              "package")
+    return "%s.%s" % (package[:dot], name)
+
+
+def import_module(name, package=None):
+    """Import a module.
+
+    The 'package' argument is required when performing a relative import. It
+    specifies the package to use as the anchor point from which to resolve the
+    relative import to an absolute import.
+
+    """
+    if name.startswith('.'):
+        if not package:
+            raise TypeError("relative imports require the 'package' argument")
+        level = 0
+        for character in name:
+            if character != '.':
+                break
+            level += 1
+        name = _resolve_name(name[level:], package, level)
+    __import__(name)
+    return sys.modules[name]