Fix bug in TargetDirAnnotation compatibility

TargetDirAnnotation was moved from firrtl to firrtl.stage. However, this
is only aliased as a val in the firrtl package object. This also needs to
be type aliased for matching against a type. This fixes a bug I ran across
in the visualizer.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>