Add caching to build.ninja generation 💾

[jmackie]

Atm we parse every source file on every ditto make in order to generate the build.ninja.

ditto/crates/ditto-make/src/build_ninja.rs

Lines 198 to 202 in a1d9631

	fn prepare_build_graph(
	sources: Sources,
	package_sources: PackageSources,
	ditto_version: &semver::Version,
	) -> Result<(BuildGraph, BuildGraphNodes)> {

This is hella inefficient. Particularly given that package sources aren't really going to change.

So I reckon we should generate a build.ninja.fragment for each package, and stitch them together on warm ditto make runs. That fragment should be invalidated/regenerated when the package sources are touched.