packaging: extract pre-computed version when running from plain tarball

The tarballs created by setup.py are not including the usual archive
markers that `hg archive` leaves, so the rewrite of the version number
computation actually left the empty version string around. This meant
that installations from PyPI would use 0.0.0 as version string.

Differential Revision: https://phab.mercurial-scm.org/D10502

setup.py

@@ -437,6 +437,9 b" elif os.path.exists('.hg_archival.txt'):"
             version = '%(latesttag)s+hg%(latesttagdistance)s.%(node).12s' % kw
     else:
         version = '0+hg' + kw.get('node', '')[:12]
+elif os.path.exists('mercurial/__version__.py'):
+    data = open('mercurial/__version__.py').read()
+    version = re.search('version = b"(.*)"', data).group(1)
 
 if version:
     versionb = version