[Feature] Ship an analysis routine accompanying pt.unify_axes_tags

This will come in handy if the sparsely attached metadata is not enough to propagate across the entire computation graph in `pt.unify_axes_tags`. The proposed routine should emit a list of arrays for the user to tag that will ensure that the remaining nodes have the required metadata.

Start by reading the implementation and callers of `pt.unify_axes_tags`, then trace how sparsely attached metadata propagates through the computation graph. Identify the analysis entry point and the array objects users can tag. Done means the routine returns a list of arrays whose tags provide the metadata required by the remaining nodes.

Written by the indexing model from the issue text.