Flatten directory structure under src/awkward/_v2/operations.

[jpivarski]

Now, instead of fully-qualified names being like ak._v2.operations.convert.to_list, they're like ak._v2.operations.to_list. The directories

• convert
• describe
• reducers
• structure

have been eliminated in v2. However, v1 was untouched.

Although this PR touches a lot of files and will be hard to merge with other PRs, the changes are straightforward and should be easy to work around: the above four submodule names are simply removed.

This PR will be auto-merged right away. You'll likely want to git merge main after it's in main.

[codecov]

Codecov Report

Merging #1467 (49e45a3) into main (de7cae8) will decrease coverage by 0.47%.
The diff coverage is 66.61%.

Impacted Files	Coverage Δ
src/awkward/_v2/_connect/numexpr.py	88.40% <ø> (ø)
...c/awkward/_v2/_connect/rdataframe/to_rdataframe.py	0.00% <0.00%> (ø)
src/awkward/_v2/operations/ak_backend.py	10.00% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_arrow.py	100.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_arrow_schema.py	50.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_buffers.py	86.13% <ø> (ø)
src/awkward/_v2/operations/ak_from_cupy.py	50.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_jax.py	50.00% <ø> (ø)
src/awkward/_v2/operations/ak_from_json_new.py	0.00% <0.00%> (ø)
src/awkward/_v2/operations/ak_from_numpy.py	100.00% <ø> (ø)
... and 109 more