Avoid installing eggs from wheels when possible

This relaxes commit ff2ee218
("Do never install from wheels") because for some eggs like plantuml,
there's no tarball.

About the implementation, no need to sort by version (as it was done)
because the list is already filtered to keep the best one.

src/zc/buildout/easy_install.py

@@ -39,7 +39,11 @@ import tempfile
 import zc.buildout
 import warnings
 
+_INF = -float('inf')
 WHL_DIST = pkg_resources.EGG_DIST + 1
+def dist_key(dist):
+    key = dist.precedence
+    return _INF if key == WHL_DIST else key
 
 warnings.filterwarnings(
     'ignore', '.+is being parsed as a legacy, non PEP 440, version')
@@ -547,7 +551,7 @@ class Installer:
         # Filter the available dists for the requirement and source flag
         dists = [dist for dist in index[requirement.project_name]
                  if ((dist in requirement)
-                     and dist.precedence != WHL_DIST and
+                     and
                      ((not source) or
                       (dist.precedence == pkg_resources.SOURCE_DIST)
                       )
@@ -589,8 +593,7 @@ class Installer:
                     ):
                     return dist
 
-        best.sort()
-        return best[-1]
+        return max(dists, key=dist_key)
 
     def _fetch(self, dist, tmp, download_cache):
         if (download_cache