Prettify soong ninja build descriptions

Descriptions currently look like:
[  0% 4/29328] cc out-soong/.intermediates/external/clang/lib/Sema/libclangSema/android_arm_armv7-a-neon_denver_static_core/obj/external/clang/lib/Sema/SemaCodeComplete.o

This is not very helpful - most of the characters are used to show the
output path, which contains useful information like target architecture,
but also contains most of the path to the source files twice, and less
useful information like the exact variant name used by soong.
Make the descriptions look like:
[  0% 3/29329] //external/clang/lib/Sema:libclangSema clang++ SemaTemplateInstantiate.cpp

This is //path/to/module:modulename tool relative/path/to/source/file

Test: builds, looks pretty
Change-Id: I3087aa7d4eb1860ef6239d77407b8b35445616d7

python/builder.go

@@ -35,7 +35,6 @@ var (
 				`$parCmd -o $parFile $parArgs && echo '#!/usr/bin/env python' | cat - $parFile > $out && ` +
 				`chmod +x $out && (rm -f $initFile; rm -f $stub; rm -f $parFile)`,
 			CommandDeps: []string{"$parCmd", "$template"},
-			Description: "build par $out",
 		},
 		"initFile", "interp", "main", "template", "stub", "parCmd", "parFile", "parArgs")
 )
@@ -78,9 +77,10 @@ func registerBuildActionForModuleFileList(ctx android.ModuleContext,
 	}
 
 	ctx.ModuleBuild(pctx, android.ModuleBuildParams{
-		Rule:      android.WriteFile,
-		Output:    fileList,
-		Implicits: files,
+		Rule:        android.WriteFile,
+		Description: "generate " + fileList.Rel(),
+		Output:      fileList,
+		Implicits:   files,
 		Args: map[string]string{
 			"content": strings.Join(content, "\n"),
 		},
@@ -126,9 +126,10 @@ func registerBuildActionForParFile(ctx android.ModuleContext,
 	}
 
 	ctx.ModuleBuild(pctx, android.ModuleBuildParams{
-		Rule:      par,
-		Output:    binFile,
-		Implicits: implicits,
+		Rule:        par,
+		Description: "python archive",
+		Output:      binFile,
+		Implicits:   implicits,
 		Args: map[string]string{
 			"initFile": initFile,
 			// the "\" isn't being interpreted by regex parser, it's being