Specify module dependency in the srcs list

* "srcs" list contains one main Rust source file,
  followed by optional dependent modules.
* A dependent module included in the "srcs" list is
  the module name prefixed with ":".
* Add a simple test.

Bug: 160331255
Test: make and manual test build dependencies on genrule modules
Change-Id: I4f079138c2599158810b6412fce81b612a3f64a4
diff --git a/rust/proc_macro.go b/rust/proc_macro.go
index 2719161..49dbd8d 100644
--- a/rust/proc_macro.go
+++ b/rust/proc_macro.go
@@ -65,7 +65,8 @@
 	fileName := procMacro.getStem(ctx) + ctx.toolchain().ProcMacroSuffix()
 	outputFile := android.PathForModuleOut(ctx, fileName)
 
-	srcPath := srcPathFromModuleSrcs(ctx, procMacro.baseCompiler.Properties.Srcs)
+	srcPath, paths := srcPathFromModuleSrcs(ctx, procMacro.baseCompiler.Properties.Srcs)
+	deps.SrcDeps = paths
 
 	procMacro.unstrippedOutputFile = outputFile