getpath: Update build directory assumptions based on python-cmake-buildsystem

This ensures the following tests pass:
- `test_sys` (`SysModuleTest.test_stdlib_dir`)
- `test_frozen` (`TestFrozen.test_unfrozen_submodule_in_frozen_package`)

Author: jcfr

Lib/test/test_getpath.py

@@ -304,7 +304,7 @@ def test_buildpath_posix(self):
         ns.add_known_xfile("/home/cpython/python")
         ns.add_known_xfile("/usr/local/bin/python")
         ns.add_known_file("/home/cpython/pybuilddir.txt", ["build/lib.linux-x86_64-9.8"])
-        ns.add_known_file("/home/cpython/Lib/os.py")
+        ns.add_known_file("/home/cpython/lib/python9.8/os.py")
         ns.add_known_dir("/home/cpython/lib-dynload")
         expected = dict(
             executable="/home/cpython/python",
@@ -316,7 +316,7 @@ def test_buildpath_posix(self):
             module_search_paths_set=1,
             module_search_paths=[
                 "/usr/local/lib/python98.zip",
-                "/home/cpython/Lib",
+                "/home/cpython/lib/python9.8",
                 "/home/cpython/build/lib.linux-x86_64-9.8",
             ],
         )
@@ -492,7 +492,7 @@ def test_symlink_buildpath_posix(self):
         ns.add_known_link("/linkfrom/python", "/home/cpython/python")
         ns.add_known_xfile("/usr/local/bin/python")
         ns.add_known_file("/home/cpython/pybuilddir.txt", ["build/lib.linux-x86_64-9.8"])
-        ns.add_known_file("/home/cpython/Lib/os.py")
+        ns.add_known_file("/home/cpython/lib/python9.8/os.py")
         ns.add_known_dir("/home/cpython/lib-dynload")
         expected = dict(
             executable="/linkfrom/python",
@@ -504,7 +504,7 @@ def test_symlink_buildpath_posix(self):
             module_search_paths_set=1,
             module_search_paths=[
                 "/usr/local/lib/python98.zip",
-                "/home/cpython/Lib",
+                "/home/cpython/lib/python9.8",
                 "/home/cpython/build/lib.linux-x86_64-9.8",
             ],
         )
@@ -799,7 +799,7 @@ def test_symlink_buildpath_macos(self):
         ns.add_known_link("/linkfrom/python", "/home/cpython/python")
         ns.add_known_xfile("/usr/local/bin/python")
         ns.add_known_file("/home/cpython/pybuilddir.txt", ["build/lib.macos-9.8"])
-        ns.add_known_file("/home/cpython/Lib/os.py")
+        ns.add_known_file("/home/cpython/lib/python9.8/os.py")
         ns.add_known_dir("/home/cpython/lib-dynload")
         expected = dict(
             executable="/linkfrom/python",
@@ -811,7 +811,7 @@ def test_symlink_buildpath_macos(self):
             module_search_paths_set=1,
             module_search_paths=[
                 "/usr/local/lib/python98.zip",
-                "/home/cpython/Lib",
+                "/home/cpython/lib/python9.8",
                 "/home/cpython/build/lib.macos-9.8",
             ],
         )

Modules/getpath.py

@@ -180,7 +180,7 @@
     STDLIB_SUBDIR = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}'
     STDLIB_LANDMARKS = [f'{STDLIB_SUBDIR}/os.py', f'{STDLIB_SUBDIR}/os.pyc']
     PLATSTDLIB_LANDMARK = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}/lib-dynload'
-    BUILDSTDLIB_LANDMARKS = ['Lib/os.py']
+    BUILDSTDLIB_LANDMARKS = [f'lib/python{VERSION_MAJOR}.{VERSION_MINOR}/os.py']
     VENV_LANDMARK = 'pyvenv.cfg'
     ZIP_LANDMARK = f'{platlibdir}/python{VERSION_MAJOR}{VERSION_MINOR}.zip'
     DELIM = ':'
@@ -510,10 +510,14 @@ def search_up(prefix, *landmarks, test=isfile):
         else:
             build_stdlib_prefix = search_up(build_prefix, *BUILDSTDLIB_LANDMARKS)
         # Always use the build prefix for stdlib
+        if os_name == 'nt':
+            lib = 'Lib'
+        else:
+            lib = f'lib/python{VERSION_MAJOR}.{VERSION_MINOR}'
         if build_stdlib_prefix:
-            stdlib_dir = joinpath(build_stdlib_prefix, 'Lib')
+            stdlib_dir = joinpath(build_stdlib_prefix, lib)
         else:
-            stdlib_dir = joinpath(build_prefix, 'Lib')
+            stdlib_dir = joinpath(build_prefix, lib)
         # Only use the build prefix for prefix if it hasn't already been set
         if not prefix:
             prefix = build_stdlib_prefix