selfhost: migrate tool sources to directory packages

Build w6a and w6l from package-main directories and expose the wcc backend through a narrow package API so w6c and wwdump no longer import implementation files. Retarget the remaining load-bearing fixtures and example sources to directory packages; retain the one intentional flat compiler collision as an explicitly composed raw unit.

test/wcc/991_w6a_ww.c

@@ -112,13 +112,11 @@ main(void)
 	if (getcwd(cwd, sizeof cwd) == NULL) return 1;
 
 	/* #93 sep layout: the selfhost tools no longer leave a single
-	 * next-to-source main.s. Self-build one real tool via the explicit
-	 * main.ww compatibility root and feed EVERY emitted
+	 * next-to-source main.s. Self-build one real directory package and
+	 * feed EVERY emitted
 	 * <tool>.sepwork/<pkg>.s as
 	 * the generated-asm corpus — the root tool code plus each dep
- * package's .s (richer than the old 3 monolithic main.s). The directory
-	 * intentionally contains package main plus legacy file-import packages,
-	 * so it is not a directory-package root. */
+ * package's .s (richer than the old 3 monolithic main.s). */
 	char stem[256], cmd[4096];
 	int fail = 0;
 	int n = 0;
@@ -126,11 +124,11 @@ main(void)
 	snprintf(stem, sizeof stem, "/tmp/wwa_%d_sh", getpid());
 	snprintf(cmd, sizeof cmd,
 	    "timeout 400 %s/ww build -o %s "
-	    "%s/selfhost/cmd/w6a/main.ww >/dev/null 2>&1",
+	    "%s/selfhost/cmd/w6a >/dev/null 2>&1",
 	    bin, stem, cwd);
 	if (runwait(cmd) != 0) {
 		fprintf(stderr,
-		    "w6a_ww FAIL: cannot sep-build selfhost/cmd/w6a/main.ww\n");
+		    "w6a_ww FAIL: cannot sep-build selfhost/cmd/w6a\n");
 		/* a failed build can leave a partial stem/.sepwork tree. */
 		fail = 1;
 		goto cleanup;