build: migrate from git submodule to CPM for dependency management

[BishopWolf]

See discussion here artyom-beilis/pytorch_dlprim#116

Basically, developers would not need git submodule anymore and everything will remain updated everywhere

Resume

• Git submodules require manual init and are a common source of "why doesn't this build?" complaints from new contributors.
• CPM provides a single-file, zero-install dependency manager for CMake projects and supports a local source cache via CPM_SOURCE_CACHE.
• Packaging via pyproject.toml makes the project installable with standard Python tooling without manual CMake invocations.