[SPARK-20343][BUILD] Force Avro 1.7.7 in sbt build to resolve build failure in...

[SPARK-20343][BUILD] Force Avro 1.7.7 in sbt build to resolve build failure in SBT Hadoop 2.6 master on Jenkins

## What changes were proposed in this pull request?

This PR proposes to force Avro's version to 1.7.7 in core to resolve the build failure as below:

```
[error] /home/jenkins/workspace/spark-master-test-sbt-hadoop-2.6/core/src/main/scala/org/apache/spark/serializer/GenericAvroSerializer.scala:123: value createDatumWriter is not a member of org.apache.avro.generic.GenericData
[error]     writerCache.getOrElseUpdate(schema, GenericData.get.createDatumWriter(schema))
[error]
```

https://amplab.cs.berkeley.edu/jenkins/view/Spark%20QA%20Test%20(Dashboard)/job/spark-master-test-sbt-hadoop-2.6/2770/consoleFull

Note that this is a hack and should be removed in the future.

## How was this patch tested?

I only tested this actually overrides the dependency.

I tried many ways but I was unable to reproduce this in my local. Sean also tried the way I did but he was also unable to reproduce this.

Please refer the comments in https://github.com/apache/spark/pull/17477#issuecomment-294094092

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17651 from HyukjinKwon/SPARK-20343-sbt.

pom.xml

@@ -142,6 +142,7 @@
     <ivy.version>2.4.0</ivy.version>
     <oro.version>2.0.8</oro.version>
     <codahale.metrics.version>3.1.2</codahale.metrics.version>
+    <!-- Keep consistent with Avro vesion in SBT build for SPARK-20343 -->
     <avro.version>1.7.7</avro.version>
     <avro.mapred.classifier>hadoop2</avro.mapred.classifier>
     <jets3t.version>0.9.3</jets3t.version>

project/SparkBuild.scala

@@ -318,8 +318,8 @@ object SparkBuild extends PomBuild {
     enable(MimaBuild.mimaSettings(sparkHome, x))(x)
   }
 
-  /* Generate and pick the spark build info from extra-resources */
-  enable(Core.settings)(core)
+  /* Generate and pick the spark build info from extra-resources and override a dependency */
+  enable(Core.settings ++ CoreDependencyOverrides.settings)(core)
 
   /* Unsafe settings */
   enable(Unsafe.settings)(unsafe)
@@ -443,6 +443,16 @@ object DockerIntegrationTests {
   )
 }
 
+/**
+ * Overrides to work around sbt's dependency resolution being different from Maven's in Unidoc.
+ *
+ * Note that, this is a hack that should be removed in the future. See SPARK-20343
+ */
+object CoreDependencyOverrides {
+  lazy val settings = Seq(
+    dependencyOverrides += "org.apache.avro" % "avro" % "1.7.7")
+}
+
 /**
  * Overrides to work around sbt's dependency resolution being different from Maven's.
  */