Add URLs from parent classloader in executable jar

$ (cd spring-boot-tools; mvn clean install -DskipTests=true)
$ (cd spring-boot-samples/spring-boot-sample-simple/; mvn clean package)
$ java -jar spring-boot-samples/spring-boot-sample-simple/target/spring-boot-sample-simple-1.1.0.BUILD-SNAPSHOT.jar

(vanilla executable jar archive: works)

$ java -cp spring-boot-samples/spring-boot-sample-simple/target/spring-boot-sample-simple-1.1.0.BUILD-SNAPSHOT.jar:spring-boot-tools/spring-boot-loader/src/test/resources/jars/app.jar org.springframework.boot.loader.JarLauncher

(jar archive plus vanilla plugin: works)

$ (cd spring-boot-samples/spring-boot-sample-simple/target; rm -rf app && mkdir $_ && cd $_ && jar -xf ../*.jar)
$ java -cp spring-boot-samples/spring-boot-sample-simple/target/app/ org.springframework.boot.loader.JarLauncher

(exploded directory: works)

$ java -cp spring-boot-tools/spring-boot-loader/s:spring-boot-tools/spring-boot-loader/src/test/resources/jars/app.jar org.springframework.boot.loader.JarLauncher

(exploded directory with plugin jar: works)

Potential fix for gh-529