Refine SpringApplication source types

Update `SpringApplication` so that the `run` methods and constructors
now require `Class<?>` arguments, rather than `Objects`. String based
sources can still be loaded, but must now be set on the `getSources()`
collections. `Package` and `Resource` types are no longer directly
supported.

This change should help IDEs offer better content assist, and will
help integrations with alternative languages such as Ceylon.

Users currently passing in Class references or using the
`spring.main.sources` property should not be affected by this change. If
an XML resource is being used, some refactoring may be required (see the
changes to `SampleSpringXmlApplication` in this commit).

Fixes gh-9170

spring-boot-samples/spring-boot-sample-xml/src/main/java/sample/xml/SampleSpringXmlApplication.java

@@ -16,6 +16,8 @@
 
 package sample.xml;
 
+import java.util.Collections;
+
 import sample.xml.service.HelloWorldService;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -24,6 +26,8 @@ import org.springframework.boot.SpringApplication;
 
 public class SampleSpringXmlApplication implements CommandLineRunner {
 
+	private static final String CONTEXT_XML = "classpath:/META-INF/application-context.xml";
+
 	@Autowired
 	private HelloWorldService helloWorldService;
 
@@ -33,7 +37,9 @@ public class SampleSpringXmlApplication implements CommandLineRunner {
 	}
 
 	public static void main(String[] args) throws Exception {
-		SpringApplication.run("classpath:/META-INF/application-context.xml", args);
+		SpringApplication application = new SpringApplication();
+		application.setSources(Collections.singleton(CONTEXT_XML));
+		application.run(args);
 	}
 
 }