htools: move TagObject from Luxi.hs to OpCodes.hs

In addition to ReqQueryTags in Luxi.hs, the TagObject ADT is also
required for the "kind" attribute of OpTagsSet and OpTagsDel, which
are coming to OpCodes.hs next. Hence, we move TagObject there, and
adjust imports accordingly.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>