move pth rename and site.py removal to buildPhase

This leaves the installCommand to be only easy_install and makes it
easy again to overwrite this.

pkgs/development/python-modules/generic/default.nix

@@ -20,20 +20,6 @@
 , installCommand ?
     ''
       easy_install --prefix="$out" .
-
-      # A pth file might have been generated to load the package from
-      # within its own site-packages, rename this package not to
-      # collide with others.
-      eapth="$out/lib/${python.libPrefix}"/site-packages/easy-install.pth
-      if [ -e "$eapth" ]; then
-          # move colliding easy_install.pth to specifically named one
-          mv "$eapth" $(dirname "$eapth")/${name}.pth
-      fi
-
-      # Remove any site.py files generated by easy_install as these
-      # cause collisions. If pth files are to be processed a
-      # corresponding site.py needs to be included in the PYTHONPATH.
-      rm -f "$out/lib/${python.libPrefix}"/site-packages/site.py*
     ''
     
 , buildPhase ? "true"
@@ -72,6 +58,20 @@ python.stdenv.mkDerivation (attrs // {
     export PYTHONPATH="$out/lib/${python.libPrefix}/site-packages:$PYTHONPATH"
     ${installCommand}
 
+    # A pth file might have been generated to load the package from
+    # within its own site-packages, rename this package not to
+    # collide with others.
+    eapth="$out/lib/${python.libPrefix}"/site-packages/easy-install.pth
+    if [ -e "$eapth" ]; then
+        # move colliding easy_install.pth to specifically named one
+        mv "$eapth" $(dirname "$eapth")/${name}.pth
+    fi
+
+    # Remove any site.py files generated by easy_install as these
+    # cause collisions. If pth files are to be processed a
+    # corresponding site.py needs to be included in the PYTHONPATH.
+    rm -f "$out/lib/${python.libPrefix}"/site-packages/site.py*
+
     ${postInstall}
   '';