[SPARK-13754] Keep old data source name for backwards compatibility

## Motivation
CSV data source was contributed by Databricks. It is the inlined version of https://github.com/databricks/spark-csv. The data source name was `com.databricks.spark.csv`. As a result there are many tables created on older versions of spark with that name as the source. For backwards compatibility we should keep the old name.

## Proposed changes
`com.databricks.spark.csv` was added to list of `backwardCompatibilityMap` in `ResolvedDataSource.scala`

## Tests
A unit test was added to `CSVSuite` to parse a csv file using the old name.

Author: Hossein <hossein@databricks.com>

Closes #11589 from falaki/SPARK-13754.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/DataSource.scala

@@ -75,7 +75,8 @@ case class DataSource(
     "org.apache.spark.sql.json" -> classOf[json.DefaultSource].getCanonicalName,
     "org.apache.spark.sql.json.DefaultSource" -> classOf[json.DefaultSource].getCanonicalName,
     "org.apache.spark.sql.parquet" -> classOf[parquet.DefaultSource].getCanonicalName,
-    "org.apache.spark.sql.parquet.DefaultSource" -> classOf[parquet.DefaultSource].getCanonicalName
+    "org.apache.spark.sql.parquet.DefaultSource" -> classOf[parquet.DefaultSource].getCanonicalName,
+    "com.databricks.spark.csv" -> classOf[csv.DefaultSource].getCanonicalName
   )
 
   /** Given a provider name, look up the data source class definition. */

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVSuite.scala

@@ -466,4 +466,14 @@ class CSVSuite extends QueryTest with SharedSQLContext with SQLTestUtils {
       df.schema.fields.map(field => field.dataType).deep ==
       Array(IntegerType, IntegerType, IntegerType, IntegerType).deep)
   }
+
+  test("old csv data source name works") {
+    val cars = sqlContext
+      .read
+      .format("com.databricks.spark.csv")
+      .option("header", "false")
+      .load(testFile(carsFile))
+
+    verifyCars(cars, withHeader = false, checkTypes = false)
+  }
 }