Aspect Subindex Builder Weirdness

Calling `toTarget` on a subindex inside an aspect is causing an odd builder failure.

```scala
import chisel3._
import chisel3.aop.injecting.InjectingAspect
import chisel3.stage.{
ChiselGeneratorAnnotation,
ChiselStage
}

class Foo extends MultiIOModule {
val in = IO(Input(UInt(2.W)))
}

class FooAspect extends InjectingAspect(
{dut: Foo => Seq(dut)},
{dut: Foo =>
dut.in(0).toTarget
}
)

(new ChiselStage)
.execute(Array("--no-run-firrtl"), Seq(new FooAspect, ChiselGeneratorAnnotation(() => new Foo)))
```

This fails with:
```
chisel3.internal.ChiselException: signalName/pathName should be called after circuit elaboration
```

https://scastie.scala-lang.org/w0bbflYoTXepYy97Vex7xA

**Type of issue**: bug report

**Impact**: unknown

**Development Phase**: request

**Please tell us about your environment:**

- Chisel 3.4.0

**What is the use case for changing the behavior?**

I think this should be possible.

Reproduce the failure from the issue's Scala example, focusing on InjectingAspect, ChiselStage, and the dut.in(0).toTarget call. Trace the builder path that reaches signalName/pathName during aspect injection; done means the example completes without the elaboration exception.

Written by the indexing model from the issue text.