Stop manually creating a src dir in the pip dependencies layer

[edmorley]

Since pip will create the directory itself if needed (which is only when there are editable mode VCS dependencies being installed).

This behaviour is already tested, via:

buildpacks-python/tests/pip_test.rs

Lines 197 to 216 in ff51b31

	// This tests that:
	// - Requirements file env var interpolation works (ie: user-provided env vars have been propagated to pip).
	// - Git from the stack image can be found (ie: the system PATH has been correctly propagated to pip).
	// - The editable mode repository clone is saved into the dependencies layer not the app dir.
	// - Compiling a source distribution package (as opposed to a pre-built wheel) works.
	// - The Python headers can be found in the `include/pythonX.Y/` directory of the Python layer.
	#[test]
	#[ignore = "integration test"]
	fn pip_editable_git_compiled() {
	TestRunner::default().build(
	default_build_config( "tests/fixtures/pip_editable_git_compiled")
	.env("WHEEL_PACKAGE_URL", "https://github.com/pypa/wheel"),
	|context| {
	assert_contains!(
	context.pack_stdout,
	"Cloning https://github.com/pypa/wheel (to revision 0.40.0) to /layers/heroku_python/dependencies/src/extension-dist"
	);
	},
	);
	}

GUS-W-16390558.