fetchPypi doesn't use python under the hood and doesn't need to be tied
to a specific version of python. Moving it to top level makes it more
consistent with other fetchers and makes code generation easier.
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.
