If the lib dir is included in the sources of bsconfig.json, please make a warning.

[MiryangJung]

I put the lib dir in bsconfig.json and run build -w, and the build was run infinitely.

    "sources": [
      { "dir": "lib", "subdirs": true }
    ],

Of course, I found the cause of the problem by reading Doc, but I think it would be good to raise the warning. It is expected that need to check lib is included in bsb_parse_sources.ml

https://user-images.githubusercontent.com/48237511/174083555-9e216dfb-199c-44b3-9dd8-3831792b9d4d.mov