[SPARK-9583] [BUILD] Do not print mvn debug messages to stdout.

This allows build/mvn to be used by make-distribution.sh.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7915 from vanzin/SPARK-9583 and squashes the following commits:

6469e60 [Marcelo Vanzin] [SPARK-9583] [build] Do not print mvn debug messages to stdout.

build/mvn

@@ -51,11 +51,11 @@ install_app() {
     # check if we have curl installed
     # download application
     [ ! -f "${local_tarball}" ] && [ $(command -v curl) ] && \
-      echo "exec: curl ${curl_opts} ${remote_tarball}" && \
+      echo "exec: curl ${curl_opts} ${remote_tarball}" 1>&2 && \
       curl ${curl_opts} "${remote_tarball}" > "${local_tarball}"
     # if the file still doesn't exist, lets try `wget` and cross our fingers
     [ ! -f "${local_tarball}" ] && [ $(command -v wget) ] && \
-      echo "exec: wget ${wget_opts} ${remote_tarball}" && \
+      echo "exec: wget ${wget_opts} ${remote_tarball}" 1>&2 && \
       wget ${wget_opts} -O "${local_tarball}" "${remote_tarball}"
     # if both were unsuccessful, exit
     [ ! -f "${local_tarball}" ] && \
@@ -146,7 +146,7 @@ fi
 # Set any `mvn` options if not already present
 export MAVEN_OPTS=${MAVEN_OPTS:-"$_COMPILE_JVM_OPTS"}
 
-echo "Using \`mvn\` from path: $MVN_BIN"
+echo "Using \`mvn\` from path: $MVN_BIN" 1>&2
 
 # Last, call the `mvn` command as usual
 ${MVN_BIN} "$@"