Don't include spring-boot-loader.jar and spring-boot-jarmode-layer-tools.jar files in the source jar of spring-boot-loader-tools

[wilkinsona]

The source jar for spring-boot-loader-tools currently includes the standard (not source) jars for spring-boot-loader and spring-boot-jarmode-layer-tools. This seems a bit pointless as the .class files in the nested jars aren't of much use to anyone who cares about the source. Furthermore, the source is already published separately in the spring-boot-loader and spring-boot-jarmode-layer-tools modules.