wcc/check: reject self-import, both stages (#16 ENFORCE-checker)

check-(c): a package importing itself (any spelling) is a hard error,
mirroring Go. Predicate is leaf==owner at the N_USE/installdecl seam —
sound only after the PREP commits (dotted-test renames, package-less
boundary directive). Identical wording both stages; diagnostics-only,
byte-id-neutral. 948 pins the reject in both compilers; 708's
pos_selfimp (which pinned the abolished self-import skip) converts to
neg_selfimp + new pos_crossmod preserving the param-shadow tolerance
the case existed for. Checks (a) unused and (b)/(d) name-membership
stay deferred to the multi-package arc: imports are filename-keyed
pulls, so those need import->file provenance this compiler lacks.

cmd/wcc/check.c

@@ -2810,6 +2810,16 @@ check_file(Checker *c, Node *file)
 	 * walked in the next pass. */
 	for (Node *d = file->list; d; d = d->next) {
 		if (d->kind == N_USE) {
+			/* check-(c) self-import: a package may not import
+			 * itself. Pure owner==leaf string compare, package-
+			 * model-independent — sound under ww's filename-keyed
+			 * file-inclusion imports. check-(a) unused and
+			 * (b)/(d) membership DEFERRED to task #8 (filename-
+			 * keyed pulls lack import->file->symbol provenance). */
+			const char *owner = decl_mod(file, d);
+			if (owner && owner[0] && strcmp(d->str, owner) == 0)
+				err(c, d->pos, "self-import: package "
+				    "'%s' cannot import itself", owner);
 			Sym *prev = scope_lookup_local(c->cur, d->str);
 			if (prev != NULL) {
 				/* Self-import: the driver concatenates the