[feat] Respect pinned versions in bootstrap

If zc.buildout or its dependencies have pinned versions that do not
match the currently running versions, they are now installed in the
local eggs directory from scratch according to the pinned versions.

In offline mode this merely ensures that versions that satisfy the
requirements are already available. This is the case when the eggs
are already installed, or when the running versions are a match to
the pinned versions or the absence of a pinned version.

If after this matching versions of zc.buildout and its dependencies
are not located in the local eggs or develop-eggs directories, they
are copied there as was already the case before.

src/zc/buildout/buildout.py

@@ -615,9 +615,29 @@ class Buildout(DictMixin):
 
         self._setup_directories()
 
+        # Install buildout and dependent eggs following pinned versions.
+        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,
+                )
+        # If versions aren't pinned or if current modules match,
+        # nothing will be installed, but then we'll copy them to
+        # the local eggs or develop-eggs folder just after this.
+
         # Now copy buildout and setuptools eggs, and record destination eggs:
         entries = []
-        for dist in zc.buildout.easy_install.buildout_and_setuptools_dists:
+        for dist in ws:
             if dist.precedence == pkg_resources.DEVELOP_DIST:
                 dest = os.path.join(self['buildout']['develop-eggs-directory'],
                                     dist.key + '.egg-link')