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/interpreters/python/hooks/pip-install-hook.sh

@@ -0,0 +1,24 @@
+# Setup hook for pip.
+echo "Sourcing pip-install-hook"
+
+declare -a pipInstallFlags
+
+pipInstallPhase() {
+    echo "Executing pipInstallPhase"
+    runHook preInstall
+
+    mkdir -p "$out/@pythonSitePackages@"
+    export PYTHONPATH="$out/@pythonSitePackages@:$PYTHONPATH"
+
+    pushd dist || return 1
+    @pythonInterpreter@ -m pip install ./*.whl --no-index --prefix="$out" --no-cache $pipInstallFlags --build tmpbuild
+    popd || return 1
+
+    runHook postInstall
+    echo "Finished executing pipInstallPhase"
+}
+
+if [ -z "$dontUsePipInstall" ] && [ -z "$installPhase" ]; then
+    echo "Using pipInstallPhase"
+    installPhase=pipInstallPhase
+fi