Support for variant-specific headers

This is common for libraries that generate header files that contain OS/tool chain/variant specific information. There may be a 'common' set of headers as well, but resolving both a common set and a variant specific set is not required for this to be completed.

- Libraries can publish headers that are specific to a variant
- Applications and other libraries can consume those headers
- Wrapped libraries should be able to publish variant specific headers as well.

The issue names no files, tests, or entry points. Start by locating the existing header publication and consumption paths, then trace how variant information is resolved. Done means libraries can publish variant-specific headers, applications and other libraries can consume them, and wrapped libraries can publish them as well.

Written by the indexing model from the issue text.