Maintain classpath order in PropertiesLauncher

I think this is safe, judging by the integration tests, but I'm not
putting it in 1.2.x until we've had some feedback on it. The
integration tests actually had a bug that was masking this problem
because they were merging Properties from the whole classpath instead
of picking the first available resource (which is generally what
we do in Spring Boot applications for application.properties for
instance).

Fixes gh-3048

spring-boot-tools/spring-boot-loader/src/it/executable-props/verify.groovy

@@ -26,7 +26,6 @@ assert out.contains('Hello Embedded Foo!'),
 	'With loader.path=.,lib should use the application.properties from the local filesystem\n' + out
 
 new File("${basedir}/target/application.properties").withWriter { it -> it << "message: Spam" }
-
 out = exec("java -Dloader.path=target,.,lib -jar ${jarfile}")
 assert out.contains('Hello Embedded Spam!'),
 	'With loader.path=target,.,lib should use the application.properties from the target directory\n' + out