Only depend on a single file for generated headers

While the rule may really need all of the generated header files to
exist, only one of them (per genrule task) needs to be in the dependency
list, since the rest are essentially aliases.

This brings an AOSP aosp_arm-userdebug out/soong/build.ninja file from
372MB to 156MB, with equivalent functionality. The Android-aosp_arm.mk
file is reduced from 11MB to 6.5MB.

Bug: 73745773
Test: diff out/soong/build.ninja
Test: diff out/soong/Android-aosp_arm.mk
Test: rm -rf out; m
Change-Id: If17377666292cc20957417fc4c3cd52f98971d0c

genrule/genrule.go

@@ -43,6 +43,7 @@ func init() {
 type SourceFileGenerator interface {
 	GeneratedSourceFiles() android.Paths
 	GeneratedHeaderDirs() android.Paths
+	GeneratedDeps() android.Paths
 }
 
 type HostToolProvider interface {
@@ -107,6 +108,7 @@ type Module struct {
 	exportedIncludeDirs android.Paths
 
 	outputFiles android.Paths
+	outputDeps  android.Paths
 }
 
 type taskFunc func(ctx android.ModuleContext, rawCommand string, srcFiles android.Paths) generateTask
@@ -130,6 +132,10 @@ func (g *Module) GeneratedHeaderDirs() android.Paths {
 	return g.exportedIncludeDirs
 }
 
+func (g *Module) GeneratedDeps() android.Paths {
+	return g.outputDeps
+}
+
 func (g *Module) DepsMutator(ctx android.BottomUpMutatorContext) {
 	android.ExtractSourcesDeps(ctx, g.properties.Srcs)
 	android.ExtractSourcesDeps(ctx, g.properties.Tool_files)
@@ -334,6 +340,7 @@ func (g *Module) generateSourceFile(ctx android.ModuleContext, task generateTask
 	for _, outputFile := range task.out {
 		g.outputFiles = append(g.outputFiles, outputFile)
 	}
+	g.outputDeps = append(g.outputDeps, task.out[0])
 }
 
 func generatorFactory(taskGenerator taskFunc, props ...interface{}) *Module {