Locate and/or download `spec` to `tmpdir`, returning a local path

        `spec` may be a ``Requirement`` object, or a string containing a URL,
        an existing local filename, or a project/version requirement spec
        (i.e. the string form of a ``Requirement`` object).  If it is the URL
        of a .py file with an unambiguous ``#egg=name-version`` tag (i.e., one
        that escapes ``-`` as ``_`` throughout), a trivial ``setup.py`` is
        automatically created alongside the downloaded file.

        If `spec` is a ``Requirement`` object or a string containing a
        project/version requirement spec, this method returns the location of
        a matching distribution (possibly after downloading it to `tmpdir`).
        If `spec` is a locally existing file or directory name, it is simply
        returned unchanged.  If `spec` is a URL, it is downloaded to a subpath
        of `tmpdir`, and the local filename is returned.  Various errors may be
        raised if a problem occurs during downloading.

def download(self, spec, tmpdir):
        """Locate and/or download `spec` to `tmpdir`, returning a local path

        `spec` may be a ``Requirement`` object, or a string containing a URL,
        an existing local filename, or a project/version requirement spec
        (i.e. the string form of a ``Requirement`` object).  If it is the URL
        of a .py file with an unambiguous ``#egg=name-version`` tag (i.e., one
        that escapes ``-`` as ``_`` throughout), a trivial ``setup.py`` is
        automatically created alongside the downloaded file.

        If `spec` is a ``Requirement`` object or a string containing a
        project/version requirement spec, this method returns the location of
        a matching distribution (possibly after downloading it to `tmpdir`).
        If `spec` is a locally existing file or directory name, it is simply
        returned unchanged.  If `spec` is a URL, it is downloaded to a subpath
        of `tmpdir`, and the local filename is returned.  Various errors may be
        raised if a problem occurs during downloading.
        """
        if not isinstance(spec, Requirement):
            scheme = URL_SCHEME(spec)
            if scheme:
                # It's a url, download it to tmpdir
                found = self._download_url(scheme.group(1), spec, tmpdir)
                base, fragment = egg_info_for_url(spec)
                if base.endswith('.py'):
                    found = self.gen_setup(found, fragment, tmpdir)
                return found
            elif os.path.exists(spec):
                # Existing file or directory, just return it
                return spec
            else:
                spec = parse_requirement_arg(spec)
        return getattr(self.fetch_distribution(spec, tmpdir), 'location', None)