Clarify the XML id uniqueness constraint of a profile.

[fosdev]

This relates to #5 in that it could be possible to dereference an external profile that results in an XML document with non-unique ids. We talked about addressing this in authoring guidelines that indicate one must check this w/r to composing other profiles, linting profiles and their external references when they are submitted to catch non-uniqueness (albeit this does not catch changes in profiles after the fact).

So, external dependencies can really mess this up at any point if they change. One other option is a dereferencing guideline that if an id is found that is non-unique, turning it into a unique id and preserving the prior id as the name of a descriptor. If the descriptor already has a name, then one would only need to make the id non-unique.

This does also relate to #3 around the use of id vs. name.

At the heart of this is what does it mean to dereference an ALPS profile, should that deref'd profile still be valid ALPS (presumably yes) and what guidelines/constraints are necessary to ensure that.