(PUP-2689) Make AST Resource check tags against "inherited" tags

This overrides the tagged? method in AST Resource to also search
for tags in its containing resource. These tags will be set when
the resource is finalized (just before serialization on transformation
to external format).

If this is not done, resources will have a different tag set when
a search is made for the node itself (exported or virtual) vs. searching
external resources.

This also correct minor problems with passing an array (which could
result in an array being set as a tag).
It was also possible for empty tags to be set if a string tag contained
multiple :: in sequence, or if ending with ::. Now an error is raised
for these conditions (just like for empty segments generated by using a
sequence of commas (which was on a different code path).