add system scope jar support

### New feature, improvement proposal

please add system scope jar support, for package one jar. the sytem scope jar is very old, it just want to merge to a new jar, now it cant support.

Start by tracing how the Maven Shade Plugin currently handles system-scope JARs and the process that merges dependencies into a new JAR; the issue names no files or tests. Done means a system-scope JAR can be merged into the new JAR, with the behavior verified by an appropriate test.

Written by the indexing model from the issue text.