[HUDI-3457] Refactored Spark DataSource Relations to avoid code duplication (#4877)

Refactoring Spark DataSource Relations to avoid code duplication. 

Following Relations were in scope:

- BaseFileOnlyViewRelation
- MergeOnReadSnapshotRelaation
- MergeOnReadIncrementalRelation

hudi-spark-datasource/hudi-spark-common/src/test/scala/org/apache/spark/execution/datasources/TestHoodieInMemoryFileIndex.scala

@@ -0,0 +1,60 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.execution.datasources
+
+import org.apache.hadoop.fs.Path
+import org.apache.spark.sql.SparkSession
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+
+import java.io.File
+import java.nio.file.Paths
+
+class TestHoodieInMemoryFileIndex {
+
+  @Test
+  def testCreateInMemoryIndex(@TempDir tempDir: File): Unit = {
+    val spark = SparkSession.builder
+      .appName("Hoodie Datasource test")
+      .master("local[2]")
+      .config("spark.serializer", "org.apache.spark.serializer.KryoSerializer")
+      .getOrCreate
+
+    val folders: Seq[Path] = Seq(
+      new Path(Paths.get(tempDir.getAbsolutePath, "folder1").toUri),
+      new Path(Paths.get(tempDir.getAbsolutePath, "folder2").toUri)
+    )
+
+    val files: Seq[Path] = Seq(
+      new Path(Paths.get(tempDir.getAbsolutePath, "folder1", "file1").toUri),
+      new Path(Paths.get(tempDir.getAbsolutePath, "folder1", "file2").toUri),
+      new Path(Paths.get(tempDir.getAbsolutePath, "folder2", "file3").toUri),
+      new Path(Paths.get(tempDir.getAbsolutePath, "folder2", "file4").toUri)
+    )
+
+    folders.foreach(folder => new File(folder.toUri).mkdir())
+    files.foreach(file => new File(file.toUri).createNewFile())
+
+    val index = HoodieInMemoryFileIndex.create(spark, Seq(folders(0), folders(1)))
+    val indexedFilePaths = index.allFiles().map(fs => fs.getPath)
+    assertEquals(files.sortWith(_.toString < _.toString), indexedFilePaths.sortWith(_.toString < _.toString))
+    spark.stop()
+  }
+
+}