(maint) Remove unused Puppet::Parser::AST::Tag class

It has been hiding in a corner for 8 years. Unused.
Now removed.
(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).
(maint) Update test to use resource matcher

The other test added for checking the tagging behavior used the resource
matcher but the current compiler spec didn't. This updates it to be
consistent with the future compiler spec and use the matcher.
(maint) Remove failure for empty generated tags

Commit 0436eb1e0997a6f19ea734923a1b82625a4034f8 added an extra failure
condition for any generated tags that end up being empty strings. This
was to get it to handle that the same way that it handles a simple empty
string tag. However, this could break any installations that generate
"::" separated tags that could create 4 ":" in a row ("a::::b"), and it
also doesn't address enough of the odd tagging behavior to make it worth
it. This commit reverts that change and adds some more tests to cover
some more of the ways in which setting tags behaves.
Merge branch 'pr/2712' into stable

* pr/2712:
  (maint) Remove failure for empty generated tags
  (maint) Update test to use resource matcher
  (PUP-2689) Make AST Resource check tags against "inherited" tags
  (maint) Remove unused Puppet::Parser::AST::Tag class