src/: Disentangle some pieces of module-parsing preparation.

src/: Disentangle some pieces of module-parsing preparation.

Have `*module-map*' initialized in strictly one place, rather than a
`defparameter' and sneakily in `test-module'.  This is now a
`clear-the-decks' operation.  Use `defvar-unbound' to leave a trap for
the incautious who try to use it before the decks are cleared.