Split buildPythonPackage into setup hooks

This commit splits the `buildPythonPackage` into multiple setup hooks.

Generally, Python packages are built from source to wheels using `setuptools`.
The wheels are then installed with `pip`. Tests were often called with
`python setup.py test` but this is less common nowadays. Most projects
now use a different entry point for running tests, typically `pytest`
or `nosetests`.

Since the wheel format was introduced more tools were built to generate these,
e.g. `flit`. Since PEP 517 is provisionally accepted, defining a build-system
independent format (`pyproject.toml`), `pip` can now use that format to
execute the correct build-system.

In the past I've added support for PEP 517 (`pyproject`) to the Python
builder, resulting in a now rather large builder. Furthermore, it was not possible
to reuse components elsewhere. Therefore, the builder is now split into multiple
setup hooks.

The `setuptoolsCheckHook` is included now by default but in time it should
be removed from `buildPythonPackage` to make it easier to use another hook
(curently one has to pass in `dontUseSetuptoolsCheck`).

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

@@ -1,25 +1,32 @@
 { lib
+, python
 , buildPythonPackage
+, bootstrapped-pip
 , fetchPypi
 , mock
 , scripttest
 , virtualenv
 , pretend
 , pytest
+, setuptools
+, wheel
 }:
 
 buildPythonPackage rec {
   pname = "pip";
   version = "19.1.1";
+  format = "other";
 
   src = fetchPypi {
     inherit pname version;
     sha256 = "44d3d7d3d30a1eb65c7e5ff1173cdf8f7467850605ac7cc3707b6064bddd0958";
   };
 
+  nativeBuildInputs = [ bootstrapped-pip ];
+
   # pip detects that we already have bootstrapped_pip "installed", so we need
   # to force it a little.
-  installFlags = [ "--ignore-installed" ];
+  pipInstallFlags = [ "--ignore-installed" ];
 
   checkInputs = [ mock scripttest virtualenv pretend pytest ];
   # Pip wants pytest, but tests are not distributed