[FIRRTL] Support dynamic indexed vector element in Target

[unlsycn]

CIRCT currently cannot resolve annotations targeting dynamically indexed (non-literal) vector elements, such as:

  {
    "class":"firrtl.transforms.DontTouchAnnotation",
    "target":"~Foo|Foo>in[idx].foo"
  }

and we should support this.
A semantic concern arises regarding whether the index expression [idx] introduces ambiguity, which this could potentially reference different signals, or we can just confines the search to it within the same module as the target vector.