Change bootstrap behavior

With this mode zc.buildout and its dependencies are installed
from scratch directly in the local eggs directory, following
the pinned versions if any apply.

In offline mode this merely ensures that the required eggs are
already installed in the local eggs and develop-eggs directory.

The previous behavior is still available by passing a non-empty
set of arguments to bootstrap, e.g. bootstrap --old-behavior.

src/zc/buildout/buildout.py

@@ -615,28 +615,48 @@ class Buildout(DictMixin):
 
         self._setup_directories()
 
-        # Now copy buildout and setuptools eggs, and record destination eggs:
-        entries = []
-        for dist in zc.buildout.easy_install.buildout_and_setuptools_dists:
-            if dist.precedence == pkg_resources.DEVELOP_DIST:
-                dest = os.path.join(self['buildout']['develop-eggs-directory'],
-                                    dist.key + '.egg-link')
-                with open(dest, 'w') as fh:
-                    fh.write(dist.location)
-                entries.append(dist.location)
-            else:
-                dest = os.path.join(self['buildout']['eggs-directory'],
-                                    os.path.basename(dist.location))
-                entries.append(dest)
-                if not os.path.exists(dest):
-                    if os.path.isdir(dist.location):
-                        shutil.copytree(dist.location, dest)
-                    else:
-                        shutil.copy2(dist.location, dest)
+        if not args:
+            # Now install buildout and dependent eggs, and record destination eggs:
+            dest = self['buildout']['eggs-directory']
+            path = [self['buildout']['develop-eggs-directory']]
+            if self.offline:
+                # Cannot install: just check requirements are already met
+                path.append(dest)
+                dest = None
+            ws = zc.buildout.easy_install.install(
+                    ['zc.buildout'],
+                    dest,
+                    links=self._links,
+                    index=self['buildout'].get('index'),
+                    path=path,
+                    newest=self.newest,
+                    allow_hosts=self._allow_hosts,
+                    )
+        else:
+            # Now copy buildout and setuptools eggs, and record destination eggs:
+            entries = []
+            for dist in zc.buildout.easy_install.buildout_and_setuptools_dists:
+                if dist.precedence == pkg_resources.DEVELOP_DIST:
+                    dest = os.path.join(self['buildout']['develop-eggs-directory'],
+                                        dist.key + '.egg-link')
+                    with open(dest, 'w') as fh:
+                        fh.write(dist.location)
+                    entries.append(dist.location)
+                else:
+                    dest = os.path.join(self['buildout']['eggs-directory'],
+                                        os.path.basename(dist.location))
+                    entries.append(dest)
+                    if not os.path.exists(dest):
+                        if os.path.isdir(dist.location):
+                            shutil.copytree(dist.location, dest)
+                        else:
+                            shutil.copy2(dist.location, dest)
+            ws = pkg_resources.WorkingSet(entries)
 
-        # Create buildout script
-        ws = pkg_resources.WorkingSet(entries)
+        # Ensure all the requirements are met
         ws.require('zc.buildout')
+
+        # Create buildout script
         options = self['buildout']
         eggs_dir = options['eggs-directory']
         develop_eggs_dir = options['develop-eggs-directory']