Name the Python extension after what it contains

[shoumikhin]

The compiled Python extension in the wheel was called _portable_lib. That name
is misleading in two ways. "Portable" is the name of a kernel set in this project,
the portable kernels, and this file does not contain them: it holds the Python
bindings for the runtime. "Lib" says nothing, since every shared object is a
library. Someone looking for the portable kernels finds the Python bindings
instead, and someone looking for the bindings has no reason to open a file named
after kernels.

Rename it to _C, which is the usual name for a package's compiled C extension
and what PyTorch calls the same thing (torch/_C.cpython-*.so). The package path
already says pybindings, as in executorch.extension.pybindings, so a name like
_pybindings would repeat itself while _C reads as "this package's C
extension".

Nothing user facing changes. Applications import the public wrapper:

from executorch.extension.pybindings import portable_lib

That wrapper re-exports everything from the compiled module and keeps its name, so
existing code continues to work. Only the private module behind it is renamed, and
it was already documented as experimental and subject to change.

Test plan:

Built a wheel and confirmed the shipped file is named for the new module:

extension/pybindings/_C.cpython-312-x86_64-linux-gnu.so

Installed that wheel into a fresh environment and checked both the public wrapper
and the private module load, then exported and ran a model through the Python
bindings:

public wrapper works: True
module name         : executorch.extension.pybindings._C
exported symbols    : 22

Also grepped the tree to confirm no build file, test, or comment still names the
old module.

[shoumikhin]

Stack from ghstack (oldest at bottom):

• -> Name the Python extension after what it contains #21816
• Ship linkable libraries in the macOS wheel #21771
• Ship the OpenVINO delegate as its own library #21770
• Build and publish CUDA wheels #21668
• Ship the CUDA delegate in the wheel #21645
• Ship the quantized kernels as their own library #21642
• Ship a C++ SDK in the wheel #21639
• Split the runtime out of the Python bindings extension #21610

[pytorch-bot]

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