As part of the work to improve the spec for where tags can be used, a number of minor issues in TagletManager and related taglets have been identified.

Together, these suggest the following:

* Provide the ability to dump info about the known taglets and where they can be used
* {@value} should be allowed in MODULE
* @hidden, @treatAsPrivate should be allowed in CONSTRUCTOR
* change SimpleTaglet.locations to a Set<Location> instead of a simple String of characters
* change the first parameter of SimpleTaglet from String to DocTree.KInd
* move SimpleTaglet.locations to BaseTaglet, such that the inXyz methods can be defined in terms of the set, with no need for any overrides in any subtypes
* provide dummy taglets for the standard tags that are currently only known by their String names
* add @uses @provides in list of standardTags
* review undocumented support for @factory