Within a multi-file unit (files sharing a qualified name), every file must
agree on kind. Handled by check_group_kind_consistency; this check is
the v0.4-style directory-level guard which now defers to it.
#1885 (DECISION D): a unit split across files carries at most one
module doc — the doc-block above its header. Two or more would have to be
joined in some file order, which readers would not expect, so each file
that carries one gets bynk.project.duplicate_module_doc, naming every
such file (paths /-joined on every platform).