Release 1.5.1 not working with plone 4.3.19

To reproduce, installed from the bundle package.

https://launchpad.net/plone/4.3/4.3.19/+download/Plone-4.3.19-UnifiedInstaller.tgz

Add Products.WindowZ to the eggs section inside buildout.cfg

Add Products.WindowZ = 1.5.1 to the versions.cfg

Run bin/buildout and then, running bin/instance fg got this error:

```
File "$HOME/Plone/buildout-cache/eggs/Products.windowZ-1.5.1-py2.7.egg/Products/windowZ/upgrades.py", line 4, in
from plone import api
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File "$HOME/Plone/zinstance/parts/instance/etc/site.zcml", line 16.2-16.23
ZopeXMLConfigurationError: File "$HOME/Plone/buildout-cache/eggs/Products.windowZ-1.5.1-py2.7.egg/Products/windowZ/configure.zcml", line 9.2-9.34
ZopeXMLConfigurationError: File "$HOME/Plone/buildout-cache/eggs/Products.windowZ-1.5.1-py2.7.egg/Products/windowZ/upgrades.zcml", line 8.2-16.8
ImportError: cannot import name api

```

It works if the version is changed to 1.5 in the versions.cfg file.
Products.WindowZ = 1.5

Reproduce the failure with the Plone 4.3.19 bundle, buildout.cfg, and versions.cfg, then compare the working 1.5 setup with 1.5.1. Start with Products/windowZ/upgrades.py and its configure.zcml and upgrades.zcml references; done means bin/instance fg starts successfully with version 1.5.1 without the ImportError.

Written by the indexing model from the issue text.