Rust: WriteFileRule instead of printf for protos

Use WriteFileRule instead of a rule that calls printf for generating the
module source file for rust_protobuf.

Bug: 171361369
Test: rust_protobuf modules compile
Change-Id: Ie83a30e19d7d09875f3d209662526783862d8ea0

rust/library.go

@@ -430,6 +430,7 @@ func (library *libraryDecorator) compile(ctx ModuleContext, flags Flags, deps Pa
 	if library.sourceProvider != nil {
 		// Assume the first source from the source provider is the library entry point.
 		srcPath = library.sourceProvider.Srcs()[0]
+		deps.srcProviderFiles = append(deps.srcProviderFiles, library.sourceProvider.Srcs()...)
 	} else {
 		srcPath, _ = srcPathFromModuleSrcs(ctx, library.baseCompiler.Properties.Srcs)
 	}