[fix] Use only ws.find in install

Replace `pkg_resources.Environment(ws.entries).best_match(req, ws)`
with `ws.find(req)`.

The first already starts by calling `ws.find(req)` to attempt to find
an already activated dist in the working set, but if none is found it
then proceeds to scan through the entries of the environment - i.e.
the entries of the working set, `ws.entries` - to activate a dist at
these locations if one matches.

This is problematic if a dist from site-packages is in the working set
at this stage: this will consider site-packages over eggs installed in
.eggs, seemingly randomly depending on what dists are requested and
whether they are found in site-packages.

Also when calling `install(['zc.buildout'])` to resolve the dists of
buildout and its dependencies currently running there are no version
pins yet, so if `zc.buildout` comes from site-packages and specific
versions of its dependencies come from `.eggs` but other versions
exist in site-packages, then the dists from site-packages would be
found instead of the actually running dists from `.eggs`.

src/zc/buildout/easy_install.py

@@ -790,10 +790,6 @@ class Installer(object):
         requirements.reverse() # Set up the stack.
         processed = {}  # This is a set of processed requirements.
         best = {}  # This is a mapping of package name -> dist.
-        # Note that we don't use the existing environment, because we want
-        # to look for new eggs unless what we have is the best that
-        # matches the requirement.
-        env = pkg_resources.Environment(ws.entries)
 
         while requirements:
             # Process dependencies breadth-first.
@@ -805,7 +801,15 @@ class Installer(object):
             dist = best.get(req.key)
             if dist is None:
                 try:
-                    dist = env.best_match(req, ws)
+                    # Note that we first attempt to find an already active dist
+                    # in the working set. This will detect version conflicts.
+                    # XXX We expressly avoid activating dists in the entries of
+                    # the current working set: they might not reflect the order
+                    # of the environment. This is not so bad when the versions
+                    # are pinned, but when calling install(['zc.buildout']), it
+                    # can come up with completely different dists than the ones
+                    # currently running.
+                    dist = ws.find(req)
                 except pkg_resources.VersionConflict as err:
                     logger.debug(
                         "Version conflict while processing requirement %s "