Catch breakage related to macro re-exports

[obi1kenobi]

#946 states that macros are always at top level. This is nearly always true in practice, but doesn't always have to be the case as conversation in #966 shows.

Action items, in order:

• update our data model and import tracking in trustfall-rustdoc-adapter to handle macro exports and their re-exports
• update lints in New lints: Breaking changes in macros #946 to include importable_path clauses, so that we catch breakage on all re-exports of macros, not just the top-level exports