pyoxidizer: add the user site to `sys.path` on macOS

This allows 3rd party extensions that are installed with `pip` to be picked up,
similar to what we do on Windows.  PyOxidizer has a bug that prevents this from
working without this extra help (see 95af358fcdfe), though it appears there's
another wrinkle here with `sys._framework` too.

I needed this to see if the problem[1] loading the keyring extension on Windows
also occurs on macOS (it doesn't).

[1] https://github.com/indygreg/PyOxidizer/issues/445

Differential Revision: https://phab.mercurial-scm.org/D11452

rust/hgcli/pyoxidizer.bzl

@@ -58,6 +58,20 b" if os.name == 'nt':"
                 'site-packages',
             )
         )
+elif sys.platform == "darwin":
+    vi = sys.version_info
+
+    def joinuser(*args):
+        return os.path.expanduser(os.path.join(*args))
+
+    # Note: site.py uses `sys._framework` instead of hardcoding "Python" as the
+    #   3rd arg, but that is set to an empty string in an oxidized binary.  It
+    #   has a fallback to ~/.local when `sys._framework` isn't set, but we want
+    #   to match what the system python uses, so it sees pip installed stuff.
+    usersite = joinuser("~", "Library", "Python",
+                        "%d.%d" % vi[:2], "lib/python/site-packages")
+
+    sys.path.append(usersite)
 import hgdemandimport;
 hgdemandimport.enable();
 from mercurial import dispatch;