hierarchy API not working for inner classes

**Type of issue**: bug report

**Impact**: API addition (no impact on existing code)

I'm considering this an addition b/c it will give the existing API the capability to deal with inner modules.

**Development Phase**: request

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**
https://scastie.scala-lang.org/debs-sifive/t9ZYvcYnRSGNuTJwNqdYVg/23

**What is the current behavior?**
```
value friends is not a member of chisel3.experimental.hierarchy.Instance[Playground.ModuleMeow#FriendList]
```
Field access does not seem to work on the inner class with `@instantiable` or `IsLookupable`.

**What is the expected behavior?**
I expect the above Scastie to compile.

**Please tell us about your environment:** See Scastie

**What is the use case for changing the behavior?**
Use case is any hierarchy API application that involves accessing fields in an inner class.

Start with the linked Scastie reproduction and the Chisel hierarchy API paths involved in @instantiable, IsLookupable, and inner classes. Confirm the expected field access behavior and make the reproduction compile; the payload does not identify specific source files or tests.

Written by the indexing model from the issue text.