Fix support for isolated build

[jcfr]

The problem we are trying to solve is the following:

With the latest pip, trying to run:

pip install -e .

twice in a row led to an error like the following:

    -- Python install dir is /home/jcfr/.virtualenvs/scikit-build-35/lib/python3.5/site-packages
    CMake Error at src/python/CMakeLists.txt:68 (find_package):
      By not providing "FindPythonExtensions.cmake" in CMAKE_MODULE_PATH this
      project has asked CMake to find a package configuration file provided by
      "PythonExtensions", but CMake did not find one.

      Could not find a package configuration file provided by "PythonExtensions"
      with any of the following names:

        PythonExtensionsConfig.cmake
        pythonextensions-config.cmake

      Add the installation prefix of "PythonExtensions" to CMAKE_PREFIX_PATH or
      set "PythonExtensions_DIR" to a directory containing one of the above
      files.  If "PythonExtensions" provides a separate development package or
      SDK, be sure it has been installed.

Indeed, during first configuration the CMAKE_MODULE_PATH cache variable was set to to /tmp/pip-build-env-2_kd457b/lib/python3.5/site-packages/skbuild/resources/cmake which point to the CMake module from the isolated build environment.

[jcfr]

Waiting this is addressed, this can be mitigated running:

pip install -e . --no-build-isolation

but using this syntax will ignore any requirements specified in pyproject.toml

[jcfr]

xref #124