Replaces references to 'files' with 'sources'

Replace references to 'files' throughout the code-base with 'sources'
following the rename of `FileOptions` to `SourceOptions`.

Also cleanup ResourceUtils a little

spring-boot-cli/src/test/java/org/springframework/boot/cli/util/ResourceUtilsTests.java

@@ -25,6 +25,8 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 
 /**
+ * Tests for {@link ResourceUtils}.
+ * 
  * @author Dave Syer
  */
 public class ResourceUtilsTests {