devpi-server: move to "by-name" for later wrapping

devpi-server is a long-running application that has python plugins.
Due to circular dependencies (the plugins import devpi-server and vice
versa), and because devpi-server is not really useful as a package,
we need to wrap it later so the user can specify plugins without
collisions or infrec.

See how the "xonsh" shell is packaged.

pkgs/top-level/python-packages.nix

@@ -3870,7 +3870,7 @@ self: super: with self; {
 
   devpi-ldap = callPackage ../development/python-modules/devpi-ldap { };
 
-  devpi-server = callPackage ../development/python-modules/devpi-server { };
+  devpi-server = callPackage ../by-name/de/devpi-server/unwrapped.nix { };
 
   devpi-web = callPackage ../development/python-modules/devpi-web { };