Account for array parameters in IndexCache

[jClugstor]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[jClugstor]

Attempt to address #4272

[jClugstor]

@AayushSabharwal the issue in #4272 was that tunable_idxs had entries like c[1], c[2], but arrsym is just c. So trying to do tunable_idxs[arrsym] = idxs failed. This fixes that particular issue but I'm not sure if it's sensible to do it this way or if it breaks anything else.