Two places to ignore non-checkpointed installs

When packaging a release, skip releases which have no checktime.  When
disting a release, skip them as well, which will avoid creating an
unnecessary .exec directory we don't want.

This will automatically skip empty installs.  The use case is the
perl5 system, where we're building for all platforms/compilers, but
then deduplicating the contents to common only, when possible.  It
would be nice to implement "efs destroy install", but that's a much
more complex solution.