Install egg from wheel only if explictly required

Egg is install from wheel the requested version ends with :whl

This replaces:
  6d9b7483 Avoid installing eggs from wheels when possible
  ff2ee218 Do never install from wheels

src/zc/buildout/easy_install.py

@@ -39,11 +39,7 @@ 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')
@@ -549,9 +545,10 @@ class Installer:
             return None
 
         # Filter the available dists for the requirement and source flag
+        wheel = getattr(requirement, 'wheel', False)
         dists = [dist for dist in index[requirement.project_name]
                  if ((dist in requirement)
-                     and
+                     and (dist.precedence == WHL_DIST) == wheel and
                      ((not source) or
                       (dist.precedence == pkg_resources.SOURCE_DIST)
                       )
@@ -593,7 +590,8 @@ class Installer:
                     ):
                     return dist
 
-        return max(dists, key=dist_key)
+        best.sort()
+        return best[-1]
 
     def _fetch(self, dist, tmp, download_cache):
         if (download_cache
@@ -772,6 +770,9 @@ class Installer:
         """Return requirement with optional [versions] constraint added."""
         constraint = self._versions.get(requirement.project_name.lower())
         if constraint:
+            wheel = constraint.endswith(':whl')
+            if wheel:
+                constraint = constraint[:-4]
             try:
                 requirement = _constrained_requirement(constraint,
                                                        requirement)
@@ -779,6 +780,8 @@ class Installer:
                 logger.info(self._version_conflict_information(
                     requirement.project_name.lower()))
                 raise
+            if wheel:
+                requirement.wheel = True
 
         return requirement