[fixup] Ignore _profile_base_location_ when computing signatures

We want two identical sections at different URL to be able to produce
same signature. This feature is useful for slapos.recipe.cmmi's
shared parts.

This commit may fixup "Support ${:_profile_base_location_}." but is
purposefuly kept separate because it concerns parts signatures which
are an orthogonal feature.

src/zc/buildout/buildout.py

@@ -1688,9 +1688,16 @@ class Options(DictMixin):
             self.buildout._parts.append(name)
 
         m = md5()
+        # _profile_base_location_ is ignored in signatures, so that two sections
+        # at different URLs can have same signature
+        _profile_base_location_ = self.get('_profile_base_location_')
         # access values through .get() instead of .items() to detect unused keys
         for key in sorted(self.keys()):
+            if key == '_profile_base_location_':
+                continue
             value = self._data.get(key, self._cooked.get(key, self._raw.get(key)))
+            if _profile_base_location_:
+                value = value.replace(_profile_base_location_, '${:_profile_base_location_}')
             m.update(('%r\0%r\0' % (key, value)).encode())
         self.items_signature = '%s:%s' % (name, m.hexdigest())