A surprisingly complex set of changes to support dynamic @auto spellings.
This was driven by the need to support writers/test.py & importers/test.py, modeled closely after Terry's original code.
All tests now pass.
g.app.atAutoNames is the set of all spellings of @auto nodes.
g.app.atFileNames is the (more static) set of spellings of all other @<file> nodes.
Importing plugins updates g.app.atAutoNames.