[SPARK-16055][SPARKR] warning added while using sparkPackages with spark-submit

## What changes were proposed in this pull request?
https://issues.apache.org/jira/browse/SPARK-16055
sparkPackages - argument is passed and we detect that we are in the R script mode, we should print some warning like --packages flag should be used with with spark-submit

## How was this patch tested?
In my system locally

Author: krishnakalyan3 <krishnakalyan3@gmail.com>

Closes #14179 from krishnakalyan3/spark-pkg.

R/pkg/R/sparkR.R

@@ -155,6 +155,10 @@ sparkR.sparkContext <- function(
 
   existingPort <- Sys.getenv("EXISTING_SPARKR_BACKEND_PORT", "")
   if (existingPort != "") {
+    if (length(packages) != 0) {
+      warning(paste("sparkPackages has no effect when using spark-submit or sparkR shell",
+                    " please use the --packages commandline instead", sep = ","))
+    }
     backendPort <- existingPort
   } else {
     path <- tempfile(pattern = "backend_port")