[Dev] GN deptree tool

I wrote a little tool (https://gitee.com/cctom/gn_helper_ohos) to look up dependency information from a GN dependency tree stored in a file. It understands OHOS build <subsystem>:<part_name> and <part_name>:<module> descriptions as well. Feedback welcome. I also added some notes on the build system (not documentation, just notes).