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-build-hook.sh

@@ -0,0 +1,42 @@
+# Setup hook to use for pip projects
+echo "Sourcing pip-build-hook"
+
+pipBuildPhase() {
+    echo "Executing pipBuildPhase"
+    runHook preBuild
+
+    mkdir -p dist
+    echo "Creating a wheel..."
+    @pythonInterpreter@ -m pip wheel --no-index --no-deps --no-clean --no-build-isolation --wheel-dir dist "$options" .
+    echo "Finished creating a wheel..."
+
+    runHook postBuild
+    echo "Finished executing pipBuildPhase"
+}
+
+pipShellHook() {
+    echo "Executing pipShellHook"
+    runHook preShellHook
+
+    # Long-term setup.py should be dropped.
+    if [ -e pyproject.toml ]; then
+      tmp_path=$(mktemp -d)
+      export PATH="$tmp_path/bin:$PATH"
+      export PYTHONPATH="$tmp_path/@pythonSitePackages@:$PYTHONPATH"
+      mkdir -p "$tmp_path/@pythonSitePackages@"
+      @pythonInterpreter@ -m pip install -e . --prefix "$tmp_path" >&2
+    fi
+
+    runHook postShellHook
+    echo "Finished executing pipShellHook"
+}
+
+if [ -z "$dontUsePipBuild" ] && [ -z "$buildPhase" ]; then
+    echo "Using pipBuildPhase"
+    buildPhase=pipBuildPhase
+fi
+
+if [ -z "$shellHook" ]; then
+    echo "Using pipShellHook"
+    shellHook=pipShellHook
+fi