pythonPackages: ensure all derivations provide python modules

This adds a test to ensure no new uses of `buildPythonApplication` can
be added to `python-packages.nix`.

Python packages can be grouped into two groups: 1) applications and 2)
packages providing importable modules. In `python-packages.nix` we only
want to have 2). 1) should be in the top-level package set.

To achieve this, all setup hooks need to be marked as being a setup hook.
For the setup hooks in the Python packages set this is done by creating
a new builder, `makePythonHook`.

Because there were issues with splicing, the file importing all the hooks
is converted to an extension. All non-packages were moved out of `python-packages.nix`
into `python-packages-base.nix`. The `keep` argument to `makeScopeWithSplicing
was cleaned up as well; there is no need to keep this one manually in sync
reducing the risk of breaking cross-compilation.

pkgs/development/interpreters/python/wrap-python.nix

@@ -1,11 +1,11 @@
 { lib
 , python
-, makeSetupHook
+, makePythonHook
 , makeWrapper }:
 
 with lib;
 
-makeSetupHook {
+makePythonHook {
       deps = makeWrapper;
       substitutions.sitePackages = python.sitePackages;
       substitutions.executable = python.interpreter;