[fix] Sign dists from python library with version

Dists in python library (e.g. site-packages) using .dist-info format
are marked as DEVELOP_DIST by setuptools. Use their version to sign
these, and not their location like actual develop dists.

src/zc/buildout/buildout.py

@@ -2181,7 +2181,8 @@ def _dists_sig(dists):
             continue
         seen.add(dist)
         location = dist.location
-        if dist.precedence == pkg_resources.DEVELOP_DIST:
+        if (dist.precedence == pkg_resources.DEVELOP_DIST
+            and location != zc.buildout.easy_install.python_lib):
             result.append(dist.project_name + '-' + _dir_hash(location))
         else:
             result.append(dist.project_name + '-' + dist.version)