Simplify detection of libnetworkcache.

src/zc/buildout/buildout.py

@@ -40,6 +40,12 @@ import zc.buildout
 import zc.buildout.download
 import zc.buildout.easy_install
 
+try:
+  import slapos.libnetworkcache
+except ImportError:
+  LIBNETWORKCACHE_ENABLED = False
+else:
+  LIBNETWORKCACHE_ENABLED = True
 
 realpath = zc.buildout.easy_install.realpath
 
@@ -345,11 +351,7 @@ class Buildout(UserDict.DictMixin):
         # Get a base working set for our distributions that corresponds to the
         # stated desires in the configuration.
         distributions = ['setuptools', 'zc.buildout']
-        try:
-          import slapos.libnetworkcache
-        except ImportError:
-          pass
-        else:
+        if LIBNETWORKCACHE_ENABLED:
           distributions.append('slapos.libnetworkcache')
         if options.get('offline') == 'true':
             ws = zc.buildout.easy_install.working_set(
@@ -375,11 +377,7 @@ class Buildout(UserDict.DictMixin):
         # Now copy buildout and setuptools eggs, and record destination eggs:
         entries = []
         copy_list = ['setuptools', 'zc.buildout']
-        try:
-          import slapos.libnetworkcache
-        except ImportError:
-          pass
-        else:
+        if LIBNETWORKCACHE_ENABLED:
           copy_list.append('slapos.libnetworkcache')
         for name in copy_list:
             r = pkg_resources.Requirement.parse(name)