Add rust-project.json generator

Because we are not relying on Cargo.toml for our crate dependencies,
we need to provide a structured file to rust-analyzer which describes
the locations of the crates. Add a generator for that purpose, similarly
to cc/compdb and cc/ccdeps.

Bug: 156395307
Test: SOONG_GEN_RUST_PROJECT=1 m nothing && \
      cat ${ANDROID_BUILD_TOP}/out/soong/rust-project.json
Change-Id: I46efe0adeddae281eaf86707504c3aa15b5e80b8

rust/testing.go

@@ -100,6 +100,7 @@ func CreateTestContext() *android.TestContext {
 		ctx.BottomUp("rust_unit_tests", TestPerSrcMutator).Parallel()
 		ctx.BottomUp("rust_begin", BeginMutator).Parallel()
 	})
+	ctx.RegisterSingletonType("rust_project_generator", rustProjectGeneratorSingleton)
 
 	return ctx
 }