deballast: drop the plugin-discovery skip, act on the findings

A target whose srcs named `load_plugins`, `iter_modules`, `entry_points` or
similar was dropped from analysis entirely - every dep exempted, no record
anywhere. `deballast-keep=<label>` does the same job per dep, is greppable, and
rots visibly via `stale_keeps`, so the blanket skip only cost coverage: it hid
findings on `//cmk/utils:utils`, `//cmk/gui:app` and other targets.

Change-Id: I8322f8299f914b422df549ea5bae7a7c76b03a1e