When using octocatalog-diff with puppet resources with deep
nested datastructures such as nested arrays of objects, octocatalog-
diff would not display diffs when array elements are modified,added or removed.

In fact, it turns out `dig_out_key` doesn't descend into arrays index
that hashdiff can produce, like for instance:
testtype::test::parameters::testparam::another-one::an-array[0]::env

`dig_out_key` would stop at `an-array` because it doesn't know that's
and array index it should try to descend into.

This patch adds the functionality for `dig_out_key` to follow array
index and descend into those nested structure.

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