[feat] Install egg from wheel only if explictly required

Egg is installed from wheel when the requested version ends with :whl

src/zc/buildout/easy_install.py

@@ -58,6 +58,8 @@ except ImportError:
 
 BIN_SCRIPTS = 'Scripts' if WINDOWS else 'bin'
 
+WHL_DIST = pkg_resources.EGG_DIST + 1
+
 warnings.filterwarnings(
     'ignore', '.+is being parsed as a legacy, non PEP 440, version')
 
@@ -487,9 +489,10 @@ class Installer(object):
             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
                      (dist.precedence == pkg_resources.SOURCE_DIST if source
                       else not (dist.precedence == pkg_resources.DEVELOP_DIST
                                 and {'setup.py', 'pyproject.toml'}.isdisjoint(
@@ -675,6 +678,9 @@ class Installer(object):
         """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)
@@ -682,6 +688,8 @@ class Installer(object):
                 logger.info(self._version_conflict_information(
                     requirement.project_name.lower()))
                 raise
+            if wheel:
+                requirement.wheel = True
 
         return requirement