stage2: handle extern lib name annotation for vars

For example, a situation like this is allowed

```zig
extern "c" var stderrp: c_int;
```

In this case, `Module.Var` wrapping `stderrp` will have `lib_name`
populated with the library name where this import is expected.
db9500a314Jakub Konka committed on 2/6/2022, 8:24:13 AMparent556f0ce
2 files changedLine totals unavailable