ww: add file-scoped import aliases

cmd/wcc/ww.h

@@ -343,11 +343,12 @@ struct Node {
 					 * let) carry it. On an N_USE node
 					 * this is the importing (owning)
 					 * module. */
-	const char	*usepath;	/* M1 #22: on an N_USE node, the full
-					 * canonical import path (`encoding.utf8`);
-					 * `str` is the declared default qualifier. Drives
-					 * the path-keyed decl_mod match and the
-						 * qualified-ref codegen hint. */
+	const char	*usesource;	/* N_USE: immutable dotted source spelling. */
+	const char	*usepath;	/* N_USE: canonical, vendor-expanded identity;
+					 * initially equal to `usesource`. */
+	const char	*usealias;	/* N_USE: explicit file-local alias, or NULL. */
+	const char	*usepkgname;	/* N_USE: imported declared package name,
+					 * independent of the visible binding in `str`. */
 	const char	*pkgname;	/* declared package name for this source/export
 						 * section; independent of canonical `module`. */
 	int		sourceid;	/* lexical source-file scope within the parsed