buildPython*: wrap setuptools in hook for catching conflicts

By default buildPython* runs a hook for detecting conflicting packages.
This hook needs pkg_resources which is part of setuptools.

Before this commit, setuptools was simply added to the build. This meant
that when setuptools was forgotten to be added to the build, the build
and installation would still succeed because of this package from the
hook. During runtime (and cross-compilation) one would notice the
missing package.

pkgs/development/interpreters/python/hooks/python-catch-conflicts-hook.sh

@@ -2,7 +2,7 @@
 echo "Sourcing python-catch-conflicts-hook.sh"
 
 pythonCatchConflictsPhase() {
-    @pythonInterpreter@ @catchConflicts@
+    PYTHONPATH="@setuptools@/@pythonSitePackages@:$PYTHONPATH" @pythonInterpreter@ @catchConflicts@
 }
 
 if [ -z "${dontUsePythonCatchConflicts-}" ]; then