[feat] Propagate libnetworkcache installation

If slapos.libnetworkcache is importable, install it in bootstrap and
in buildout upgrade - the places where bin/buildout is (re)generated
- as though it were a dependency of zc.buildout.

This is a hack to propagate libnetworkcache as a soft dependency.

src/zc/buildout/buildout.py

@@ -718,6 +718,14 @@ class Buildout(DictMixin):
 
         self._setup_directories()
 
+        # Hack: propagate libnetworkcache soft dependency
+        specs = ['zc.buildout']
+        try:
+            import slapos.libnetworkcache
+            specs.append('slapos.libnetworkcache')
+        except ImportError:
+            pass
+
         # Install buildout and dependent eggs following pinned versions.
         dest = self['buildout']['eggs-directory']
         path = [self['buildout']['develop-eggs-directory']]
@@ -726,7 +734,7 @@ class Buildout(DictMixin):
             path.append(dest)
             dest = None
         ws = zc.buildout.easy_install.install(
-                ['zc.buildout'],
+                specs,
                 dest,
                 links=self._links,
                 index=self['buildout'].get('index'),
@@ -1304,8 +1312,17 @@ class Buildout(DictMixin):
         if not self.newest:
             return
 
+        # Hack: propagate libnetworkcache soft dependency
+        # XXX just zc.buildout should suffice, then iter over projects in ws
+        specs = ['zc.buildout', 'setuptools', 'pip', 'wheel']
+        try:
+            import slapos.libnetworkcache
+            specs.append('slapos.libnetworkcache')
+        except ImportError:
+            pass
+
         ws = zc.buildout.easy_install.install(
-            ('zc.buildout', 'setuptools', 'pip', 'wheel'),
+            specs,
             self['buildout']['eggs-directory'],
             links = self['buildout'].get('find-links', '').split(),
             index = self['buildout'].get('index'),
@@ -1315,7 +1332,7 @@ class Buildout(DictMixin):
 
         upgraded = []
 
-        for project in 'zc.buildout', 'setuptools', 'pip', 'wheel':
+        for project in specs:
             req = pkg_resources.Requirement.parse(project)
             dist = ws.find(req)
             importlib.import_module(project)