Sema: make check for namespace lookup of private declarations more strict

Previously sema only checked that the private declaration was in the same
file as the lookup but now it also checks that the namespace where
the decl was included from was also in the same file.

Closes #13077

test/behavior/usingnamespace.zig

@@ -75,3 +75,7 @@ test {
     const a = AA.b(42);
     try expect(a.x == AA.c().expected);
 }
+
+comptime {
+    _ = @import("usingnamespace/file_1.zig");
+}