deballast: analyze targets whose srcs include a conftest.py

A conftest is unanalyzable as a dep - pytest loads it by filename, so nothing
imports it - but as an analysis root its own imports are ordinary evidence. Only
the dep-side skip remains.

* Unused entries are dropped.

* Deps that only reached the target transitively are declared directly.
  //tests/unit/cmk/gui:conftest and others leaned on //tests/testlib/unit for
  tests.testlib.gui.*, which //tests/testlib/gui provides.

* Runtime-loaded deps are exempted with deballast-keep.

Change-Id: I02cd9ede750ff67e08c0d203ee25d00550e7fffd