Removing develop-eggs/ directory before bootstrapping.

There is code in buildout to remove develop-eggs that it knows about.
So if everything is OK, develop-egg removal works fine.
If there's something fishy goign on, however, buildout
doesn't clean it up enough. Zapping the entire directory upon bootstrap
is a very effective way to prevent problems.

Reason: the old osc.recipe.sysegg did add develop-eggs .egg-link files to the
site-packages dir, effectively short-circuiting buildout's picked versions.
Likewise an old bootstrap could have left a setuptoos.egg-link to an ancient
setuptools version. I just this minute had to help a colleague with just such
a problem.

src/zc/buildout/buildout.py

@@ -391,6 +391,12 @@ class Buildout(DictMixin):
     def bootstrap(self, args):
         __doing__ = 'Bootstrapping.'
 
+        if os.path.exists(self['buildout']['develop-eggs-directory']):
+            if os.path.isdir(self['buildout']['develop-eggs-directory']):
+                rmtree(self['buildout']['develop-eggs-directory'])
+                self._logger.debug(
+                    "Removed existing develop-eggs directory")
+
         self._setup_directories()
 
         # Now copy buildout and setuptools eggs, and record destination eggs:

src/zc/buildout/buildout.txt

@@ -2860,6 +2860,7 @@ or paths to use:
     >>> remove('setup.cfg')
     >>> print_(system(buildout + ' -csetup.cfg init demo other ./src'), end='')
     Creating '/sample-bootstrapped/setup.cfg'.
+    Creating directory '/sample-bootstrapped/develop-eggs'.
     Getting distribution for 'zc.recipe.egg>=2.0.0a3'.
     Got zc.recipe.egg
     Installing py.
@@ -2918,6 +2919,7 @@ for us:
     >>> remove('setup.cfg')
     >>> print_(system(buildout + ' -csetup.cfg init demo other ./src'), end='')
     Creating '/sample-bootstrapped/setup.cfg'.
+    Creating directory '/sample-bootstrapped/develop-eggs'.
     Installing py.
     Generated script '/sample-bootstrapped/bin/demo'.
     Generated script '/sample-bootstrapped/bin/distutilsscript'.