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/setuptools-build-hook.sh

@@ -0,0 +1,47 @@
+# Setup hook for setuptools.
+echo "Sourcing setuptools-build-hook"
+
+setuptoolsBuildPhase() {
+    echo "Executing setuptoolsBuildPhase"
+    local args
+    runHook preBuild
+
+    cp -f @setuppy@ nix_run_setup
+    args=""
+    if [ -n "$setupPyGlobalFlags" ]; then
+        args+="$setupPyGlobalFlags"
+    fi
+    if [ -n "$setupPyBuildFlags" ]; then
+        args+="build_ext $setupPyBuildFlags"
+    fi
+    eval "@pythonInterpreter@ nix_run_setup $args bdist_wheel"
+
+    runHook postBuild
+    echo "Finished executing setuptoolsInstallPhase"
+}
+
+setuptoolsShellHook() {
+    echo "Executing setuptoolsShellHook"
+    runHook preShellHook
+
+    if test -e setup.py; then
+        tmp_path=$(mktemp -d)
+        export PATH="$tmp_path/bin:$PATH"
+        export PYTHONPATH="@pythonSitePackages@:$PYTHONPATH"
+        mkdir -p "$tmp_path/@pythonSitePackages@"
+        eval "@pythonInterpreter@ -m pip -e . --prefix $tmp_path >&2"
+    fi
+
+    runHook postShellHook
+    echo "Finished executing setuptoolsShellHook"
+}
+
+if [ -z "$dontUseSetuptoolsBuild" ] && [ -z "$buildPhase" ]; then
+    echo "Using setuptoolsBuildPhase"
+    buildPhase=setuptoolsBuildPhase
+fi
+
+if [ -z "$dontUseSetuptoolsShellHook" ] && [ -z "$shellHook" ]; then
+    echo "Using setuptoolsShellHook"
+    shellHook=setuptoolsShellHook
+fi