[feat] Install libnetworkcache in bootstrap

If slapos.libnetworkcache is importable, install it in bootstrap
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

@@ -615,6 +615,14 @@ 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']]
@@ -623,7 +631,7 @@ 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'),