contrib/packaging/wix/dist.wxs

tests: look for ensurepip before using venv

Debian appears to cripple the venv module by default by
removing the associated ensurepip functionality. (The module
isn't present at all.) This caused test-install.t to fail when
using the Debian python3 unless the python3-venv package was
installed.

This commit introduces a new hghave requirement for detecting
ensurepip and makes the Python 3 install variant conditional on
its presence. This should make test-install.t pass when
using an incomplete Debian Python.

Differential Revision: https://phab.mercurial-scm.org/D7228

<?xml version="1.0" encoding="utf-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">

  <?include guids.wxi ?>
  <?include defines.wxi ?>

  <Fragment>
    <DirectoryRef Id="INSTALLDIR" FileSource="$(var.SourceDir)">
      <Component Id="distOutput" Guid="$(var.dist.guid)" Win64='$(var.IsX64)'>
        <File Name="python27.dll" KeyPath="yes" />
      </Component>
    </DirectoryRef>
  </Fragment>

</Wix>