fix: don't call asarray on Index objects internally

[agoose77]

This PR follows #2740. I've updated that PR's description for context, but let me restart the problem here!

nplike.asarray ultimately invokes NumPy's array interface mechanisms when it acts upon an Index or Content object. If we don't make this PR change, an array with placeholders that hits this codepath will lead to an exception being thrown.

As outlined in #2740, we could update nplike.asarray to handle these types, but I'm really in favour of having our internals be strict where possible; both to do less work, but also to make it easier to reason about.

So, this PR replaces any as-yet uncaught uses of nplike.asarray(index) with index.data if the nplike is unchanged, or nplike.asarray(index.data) if not.

I haven't been exhaustive; anything that isn't being run in our test suite won't have been caught, but I'm happy enough with our coverage that those will hopefully not be too many that we can't deal with them as they come in.

I haven't added anything to the test suite to catch this. I'm slightly reluctant to e.g. add a strict-mode for our test suite that throws an exception if Index._array_interface_ is inspected. We could do this, though, and I'd welcome @jpivarski thoughts!

[agoose77]

@jpivarski how do you feel about this PR?

Namely — considering nplike.asarray(index) an "error" internally, expecting it to fail under placeholders, but not adding any tests for this yet

Upfront it means we'll likely see exceptions that we could avoid by being more permissive in nplike.asarray, but I hope that this is just a bathtub model curve, and we'll have cleaner internals down the road.

[codecov]

Codecov Report

Merging #2749 (cb8c70b) into main (a6e426e) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 78.78%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/contents/indexedarray.py	78.99% <100.00%> (ø)
src/awkward/contents/indexedoptionarray.py	88.48% <100.00%> (ø)
src/awkward/contents/unmaskedarray.py	74.40% <100.00%> (ø)
src/awkward/operations/ak_flatten.py	93.47% <100.00%> (ø)
src/awkward/operations/ak_run_lengths.py	90.12% <100.00%> (ø)
src/awkward/operations/ak_to_dataframe.py	90.55% <100.00%> (-0.08%)	⬇️
src/awkward/operations/ak_to_categorical.py	93.54% <50.00%> (ø)
src/awkward/operations/ak_unflatten.py	96.15% <75.00%> (ø)
...rc/awkward/operations/ak_merge_union_of_records.py	87.03% <68.75%> (+1.45%)	⬆️

... and 1 file with indirect coverage changes

[jpivarski]

To make our tests more robust against the possibility of accidentally introducing dependence on Index.__array__ in the future, without forbidding it among end-users, we could add a private (L3) global setting that makes Index.__array__ an error when that global setting is turned on (default is off). The tests could then turn it on and be more strict than user code.

For this PR specifically, it all looks good.