rust: Cache crateRootPath to avoid ctx

This makes it possible to call crateRootPath in situations where a
ModuleContext is unavailable.

Test: m nothing
Bug: 309943184
Change-Id: Iee20b0606954a18ca516cdac40917d0016f94a05
diff --git a/rust/proc_macro.go b/rust/proc_macro.go
index e3a48bf..8a0543d 100644
--- a/rust/proc_macro.go
+++ b/rust/proc_macro.go
@@ -79,7 +79,7 @@
 	fileName := procMacro.getStem(ctx) + ctx.toolchain().ProcMacroSuffix()
 	outputFile := android.PathForModuleOut(ctx, fileName)
 
-	srcPath := procMacro.crateRootPath(ctx)
+	srcPath := crateRootPath(ctx, procMacro)
 	ret := TransformSrctoProcMacro(ctx, srcPath, deps, flags, outputFile)
 	procMacro.baseCompiler.unstrippedOutputFile = outputFile
 	return ret