chore: only copy non-test headers

[agoose77]

Whilst checking our wheel to ensure that the necessary headers are included (c.f. #2892), I noticed that an extra header file has snuck into the archive.

This PR ensures we only flatten the header-only/*/awkward/* heirarchy.

[codecov]

Codecov Report

Merging #2893 (da5cfef) into main (e2510ca) will decrease coverage by 0.01%.
Report is 3 commits behind head on main.
The diff coverage is 81.81%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/_nplikes/cupy.py	37.77% <100.00%> (ø)
src/awkward/_nplikes/jax.py	82.60% <100.00%> (ø)
src/awkward/_nplikes/numpy.py	92.10% <100.00%> (ø)
src/awkward/_nplikes/typetracer.py	74.67% <100.00%> (ø)
src/awkward/_parameters.py	82.22% <100.00%> (ø)
src/awkward/_singleton.py	89.65% <100.00%> (ø)
src/awkward/_typing.py	92.85% <100.00%> (ø)
src/awkward/operations/ak_concatenate.py	85.14% <100.00%> (ø)
src/awkward/operations/ak_from_json.py	93.57% <100.00%> (ø)
src/awkward/_connect/pyarrow.py	90.57% <66.66%> (-0.58%)	⬇️

[jpivarski]

If it was flattening the hierarchy of all *.h files, I don't see how awkward-cpp would have been compiling. The #include statements use relative subdirectories. For example,

awkward/awkward-cpp/src/libawkward/io/json.cpp

Lines 16 to 19 in ae5923e

	#include "awkward/builder/ArrayBuilder.h"
	#include "awkward/common.h"
	#include "awkward/io/json.h"

Or was this just an issue in the header files included in the sdist?

Anyway, it's a good thing to fix; please do merge.

[agoose77]

@jpivarski these are headers in the header-only part of awkward rather than awkward-cpp. For our header-only files, each component lives under awkward/XXX.h, so foo/awkward/FOO.h and bar/awkward/BAR.h can be flattened under the same awkward/ prefix