[SPARK-20590][SQL] Use Spark internal datasource if multiples are found for the same shorten name

## What changes were proposed in this pull request?

One of the common usability problems around reading data in spark (particularly CSV) is that there can often be a conflict between different readers in the classpath.

As an example, if someone launches a 2.x spark shell with the spark-csv package in the classpath, Spark currently fails in an extremely unfriendly way (see databricks/spark-csv#367):

```bash
./bin/spark-shell --packages com.databricks:spark-csv_2.11:1.5.0
scala> val df = spark.read.csv("/foo/bar.csv")
java.lang.RuntimeException: Multiple sources found for csv (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat, com.databricks.spark.csv.DefaultSource15), please specify the fully qualified class name.
  at scala.sys.package$.error(package.scala:27)
  at org.apache.spark.sql.execution.datasources.DataSource$.lookupDataSource(DataSource.scala:574)
  at org.apache.spark.sql.execution.datasources.DataSource.providingClass$lzycompute(DataSource.scala:85)
  at org.apache.spark.sql.execution.datasources.DataSource.providingClass(DataSource.scala:85)
  at org.apache.spark.sql.execution.datasources.DataSource.resolveRelation(DataSource.scala:295)
  at org.apache.spark.sql.DataFrameReader.load(DataFrameReader.scala:178)
  at org.apache.spark.sql.DataFrameReader.csv(DataFrameReader.scala:533)
  at org.apache.spark.sql.DataFrameReader.csv(DataFrameReader.scala:412)
  ... 48 elided
```

This PR proposes a simple way of fixing this error by picking up the internal datasource if there is single (the datasource that has "org.apache.spark" prefix).

```scala
scala> spark.range(1).write.format("csv").mode("overwrite").save("/tmp/abc")
17/05/10 09:47:44 WARN DataSource: Multiple sources found for csv (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat,
com.databricks.spark.csv.DefaultSource15), defaulting to the internal datasource (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat).
```

```scala
scala> spark.range(1).write.format("Csv").mode("overwrite").save("/tmp/abc")
17/05/10 09:47:52 WARN DataSource: Multiple sources found for Csv (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat,
com.databricks.spark.csv.DefaultSource15), defaulting to the internal datasource (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat).
```

## How was this patch tested?

Manually tested as below:

```bash
./bin/spark-shell --packages com.databricks:spark-csv_2.11:1.5.0
```

```scala
spark.sparkContext.setLogLevel("WARN")
```

**positive cases**:

```scala
scala> spark.range(1).write.format("csv").mode("overwrite").save("/tmp/abc")
17/05/10 09:47:44 WARN DataSource: Multiple sources found for csv (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat,
com.databricks.spark.csv.DefaultSource15), defaulting to the internal datasource (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat).
```

```scala
scala> spark.range(1).write.format("Csv").mode("overwrite").save("/tmp/abc")
17/05/10 09:47:52 WARN DataSource: Multiple sources found for Csv (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat,
com.databricks.spark.csv.DefaultSource15), defaulting to the internal datasource (org.apache.spark.sql.execution.datasources.csv.CSVFileFormat).
```

(newlines were inserted for readability).

```scala
scala> spark.range(1).write.format("com.databricks.spark.csv").mode("overwrite").save("/tmp/abc")
```

```scala
scala> spark.range(1).write.format("org.apache.spark.sql.execution.datasources.csv.CSVFileFormat").mode("overwrite").save("/tmp/abc")
```

**negative cases**:

```scala
scala> spark.range(1).write.format("com.databricks.spark.csv.CsvRelation").save("/tmp/abc")
java.lang.InstantiationException: com.databricks.spark.csv.CsvRelation
...
```

```scala
scala> spark.range(1).write.format("com.databricks.spark.csv.CsvRelatio").save("/tmp/abc")
java.lang.ClassNotFoundException: Failed to find data source: com.databricks.spark.csv.CsvRelatio. Please find packages at http://spark.apache.org/third-party-projects.html
...
```

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17916 from HyukjinKwon/datasource-detect.