Restore "all" target in subdirectories marked EXCLUDE_FROM_ALL

The "all" target in each directory is supposed to have targets from that
directory even if the directory itself is marked `EXCLUDE_FROM_ALL` in
its parent.  This was broken by commit dc688857 (Pass EXCLUDE_FROM_ALL
from directory to targets, 2019-01-15, v3.14.0-rc1~83^2) which made the
participation of a target in "all" independent of context.  Revert much
of the logic change from that commit to restore the old behavior.  Then
re-implement the behavior intended by the commit to keep its test
working.  Extend the test to cover the old behavior too.

Fixes: #19753