Fix: _pack_layout should also pack projected index arrays

[agoose77]

When _pack_layout is passed an indexed array, it returns layout.project(). However, _pack_layout is supposed to return a layout whereby only its children may need packing. In this case, by returning layout.project(), we are not guaranteed to satisfy this expectation.

This PR fix the issue by ensuring that we pack the result of project.

[codecov]

Codecov Report

Merging #1195 (02c64e3) into main (9955401) will increase coverage by 1.53%.
The diff coverage is 83.23%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/pyarrow.py	83.33% <0.00%> (-0.48%)	⬇️
src/awkward/_v2/_reducers.py	96.48% <ø> (ø)
src/awkward/_v2/forms/recordform.py	66.46% <0.00%> (ø)
src/awkward/_v2/operations/convert/ak_to_numpy.py	100.00% <ø> (ø)
...c/awkward/_v2/operations/structure/ak_unflatten.py	80.00% <ø> (ø)
src/awkward/_v2/types/arraytype.py	80.76% <0.00%> (-0.72%)	⬇️
src/awkward/_v2/contents/unmaskedarray.py	56.27% <51.72%> (+0.82%)	⬆️
src/awkward/_v2/contents/indexedarray.py	59.75% <63.23%> (+0.44%)	⬆️
src/awkward/_v2/_typetracer.py	69.19% <63.58%> (-0.36%)	⬇️
src/awkward/_v2/contents/emptyarray.py	69.54% <74.19%> (+0.26%)	⬆️
... and 31 more

[jpivarski]

ak.packed has already been translated to v2; is this issue present in the translated function? We don't want the bug to regress when we update. Thanks!

[agoose77]

This should already be fixed in v2 after looking at the source: we immediately call packed() on the project()ed result

[jpivarski]

Great! Then if you think it's done, you can go ahead and "squash and merge" it.

(I just checked: you should have access to do so.)