python setup.py develop ---help displays them, with use messaes

Options are

   --level-zerop-support=(true|false)
   --coverage=(true|false)
   --sycl-compiler-path=(path to prefix, optional)

If path is not provided backend builder looks up ONEAPI_ROOT variable.

In the path, builder looks for bin/dpcpp (Linux), or bin/dpcpp.exe (Windows).
If found, cmake does see -DDPCTL_DPCPP_ROOT variable set.

Otherwise the variable is set, and clang++ is used instead of dpcpp

CODE_COVERAGE was used to influence whether the backend SyclInterface library
was built in debug mode with coverage over dpctp-capi/ test suite collected or not.

It was also influencing how Cython extensions were built. It would pass linetrace=true
option to cythonize, and define CYTHON_TRACE preprocessor variable when compiling
Cython-generated C++ source files with compiler.

This change removes use CODE_COVERAGE all together. Instead:

1. Custom setuptools command 'build_ext' is implemented which auto-adds
   'CYTHON_TRACE' to the list of defines received by downstream build_ext
   command if develop command received --coverage=True

   cythonize call was removed from extensions() function, allowing
   ``python setup.py develop --help` execute cleanly without running
   cythonize.

   Consequentially, linetrace Cython directive will need to be added to
   each .pyx file in a separate commit.  This is safe to do per

   https://cython.readthedocs.io/en/latest/src/tutorial/profiling_tutorial.html#enabling-line-tracing

   since it is a no-op unless CYTHON_TRACE preprocessor variable is also set,
   which is only done when --coverage=True is used

2. install setuptools command removed support for `--coverage` option, and always
   builds backend without coverage.

   This is to avoid inadvertent installation of debug build of library in Python prefix.