scripts: sphinx-pre-install: rework the sphinx install logic

The sphinx-pre-install supports installing sphinx via a virtual
environment using pip/pypi or directly from the distribution's
package, when --no-virtualenv is used.

However, even when --no-virtualenv, the current logic is
still recomending to install a virtual env, due to a regression.

It turns that the logic there is complex, as it depends on
several different conditions.

Split the code which recommends Sphinx on two separate
functions, in order to clean up the code.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9dedaec201803017b7a7dc24a074f3a4f040b72a.1621949137.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>