An extensionless import path that names a directory next to a same-named
.sx file ('modules/std' with both modules/std.sx and modules/std/ present)
no longer silently resolves to the directory — it errors and asks for the
explicit .sx spelling. Exemption: a file importing its own companion
directory (X.sx importing X/, the multi-file test layout) stays legal —
the sibling file is the importer itself, so the directory is the only
sensible target.
