Don't use `sycl::detail` namespace

[AlexeySachkov]

sycl::detail::pi is used in:

https://github.com/oneapi-src/oneMKL/blob/6c5f7ea783a7fe828c0de3c064c3bc837727524d/src/blas/backends/cublas/cublas_scope_handle.cpp#L123

Please note there detail sub-namespace is reserved for implementations and there is no guarantees about its content. Not only that is non-portable, but it is also unmaintainable, because there is always risk that internal implementation details will change tomorrow.

[andrewtbarker]

@AlexeySachkov Thank you for reporting this. Do you have a suggestion for how to accomplish what we are doing here in some other way?

[AlexeySachkov]

Hi @andrewtbarker, unfortunately I don't see any core or an extension-provided feature which would expose the same functionality. Therefore, it seems to me that you may need to request a corresponding extension to be added to have this API formalized - this won't be portable still, but at least it would not be relying on implementation details.