Move all trace exporters in exporters/trace directory. (#719)

diff --git a/settings.gradle b/settings.gradle
index d236d2b..00ee47f 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -24,9 +24,11 @@
 project(':opencensus-contrib-agent').projectDir = "$rootDir/contrib/agent" as File
 project(':opencensus-contrib-grpc-metrics').projectDir = "$rootDir/contrib/grpc_metrics" as File
 project(':opencensus-contrib-grpc-util').projectDir = "$rootDir/contrib/grpc_util" as File
-project(':opencensus-exporter-trace-logging').projectDir = "$rootDir/exporters/trace_logging" as File
-project(':opencensus-exporter-trace-stackdriver').projectDir = "$rootDir/exporters/trace_stackdriver" as File
-project(':opencensus-exporter-trace-zipkin').projectDir = "$rootDir/exporters/trace_zipkin" as File
+project(':opencensus-exporter-trace-logging').projectDir =
+        "$rootDir/exporters/trace/logging" as File
+project(':opencensus-exporter-trace-stackdriver').projectDir =
+        "$rootDir/exporters/trace/stackdriver" as File
+project(':opencensus-exporter-trace-zipkin').projectDir = "$rootDir/exporters/trace/zipkin" as File
 
 // Java8 projects only
 if (JavaVersion.current().isJava8Compatible()) {