vndk-def: Split eligible list checker

This commit split eligible list checker from `check-dep` subcommand.
Eligible list checker is only an internal tool to review the eligible
vndk list.  Device vendors don't have to run it.

Test: Eligible list violations (e.g. libgui.so) no longer show up in
the output of `check-dep` command.

Change-Id: I993dcced5e3fe8bd84784b3c9b8345fa9f02e339