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`.