hg: add user-site to `sys.path` on Windows to allow pip-installed extensions

This has been in the TortoiseHg builds for several cycles now on Windows, and
even longer on macOS.  It allows an extension to be configured with `ext =`
syntax, instead of requiring the full path to be specified.  It's confusing for
a user to be hit with messages about not being able to load extensions, based
solely on which `hg.exe` is being run.

This only applies to py2exe binaries, since wrapper.exe already sees into the
user site area.  There are no frozen binaries on other platforms (that I'm aware
of), and an equivalent change will need to be made to `dispatch.py` in order to
work with PyOxidizer, since it bypasses this module completely.  (It also has
the ability to use the `site` module, so it will look completely different.)

Differential Revision: https://phab.mercurial-scm.org/D9531

hg

@@ -21,6 +21,17 b" if libdir != '@' 'LIBDIR' '@':"
         libdir = os.path.abspath(libdir)
     sys.path.insert(0, libdir)
 
+# Make `pip install --user ...` packages available to the official Windows
+# build.  Most py2 packaging installs directly into the system python
+# environment, so no changes are necessary for other platforms.  The Windows
+# py2 package uses py2exe, which lacks a `site` module.  Hardcode it according
+# to the documentation.
+if getattr(sys, 'frozen', None) == 'console_exe':
+    vi = sys.version_info
+    sys.path.append(os.path.join(os.environ['APPDATA'], 'Python',
+                                 'Python%d%d' % (vi[0], vi[1]),
+                                 'site-packages'))
+
 from hgdemandimport import tracing
 
 with tracing.log('hg script'):