pyoxidizer: add user-site to `sys.path` on Windows

This is a port of 53221078e0de to Windows to allow pip-installed extensions to
be loaded without specifying a path.  It's a major headache to have an hg.exe on
`PATH` that needs to have the path to the extensions specified, because WSL
doesn't see the same path.

This is only for Windows for now, to match the currently shipping py2 behavior.
There is a better solution with using the `site` package, but this needs support
in PyOxidizer[1].

[1] https://github.com/indygreg/PyOxidizer/issues/430

Differential Revision: https://phab.mercurial-scm.org/D11308

rust/hgcli/pyoxidizer.bzl

@@ -44,6 +44,17 b' if extra_path is not None:'
     # We do not prepend the values because the Mercurial library wants to be in
     # the front of the sys.path to avoid picking up other installations.
     sys.path.extend(extra_path.split(os.pathsep))
+# Add user site to sys.path to load extensions without the full path
+if os.name == 'nt':
+    vi = sys.version_info
+    sys.path.append(
+        os.path.join(
+            os.environ['APPDATA'],
+            'Python',
+            'Python%d%d' % (vi[0], vi[1]),
+            'site-packages',
+        )
+    )
 import hgdemandimport;
 hgdemandimport.enable();
 from mercurial import dispatch;