scripts: sphinx-pre-install: improve Gentoo package deps logic

It took me a lot of time, but I guess understand now what it
takes to install a package on Gentoo.

Handling dependencies is a nightmare, as Gentoo refuses to emerge
some packages if there's no package.use file describing them.
To make it worse, compilation flags shall also be present there
for some packages. If USE is not perfect, error/warning messages
like those are shown:

  gnome-base/librsvg dev-texlive/texlive-xetex media-fonts/dejavu dev-python/pyyaml

...

!!! The following binary packages have been ignored due to non matching USE:

    =media-gfx/graphviz-12.2.1-r1 X pdf -python_single_target_python3_13 qt6 svg
    =media-gfx/graphviz-12.2.1-r1 X pdf python_single_target_python3_12 -python_single_target_python3_13 qt6 svg
    =media-gfx/graphviz-12.2.1-r1 X pdf qt6 svg
    =media-gfx/graphviz-12.2.1-r1 X pdf -python_single_target_python3_10 qt6 svg
    =media-gfx/graphviz-12.2.1-r1 X pdf -python_single_target_python3_10 python_single_target_python3_12 -python_single_target_python3_13 qt6 svg
    =media-fonts/noto-cjk-20190416 X
    =app-text/texlive-core-2024-r1 X cjk -xetex
    =app-text/texlive-core-2024-r1 X -xetex
    =app-text/texlive-core-2024-r1 -xetex
    =dev-libs/zziplib-0.13.79-r1 sdl

If emerge is allowed, it will simply ignore the above packages,
creating an incomplete installation, which will later fail when
one tries to build docs with images or build PDFs.

After the fix, command line commands to produce the needed USE
chain will be emitted, if they don't exist yet.

        sudo su -c 'echo "media-gfx/graphviz" > /etc/portage/package.use/graphviz'
        sudo su -c 'echo "media-gfx/imagemagick" > /etc/portage/package.use/imagemagick'
        sudo su -c 'echo "media-libs/harfbuzz icu" > /etc/portage/package.use/media-libs'
        sudo su -c 'echo "media-fonts/noto-cjk" > /etc/portage/package.use/media-fonts'
        sudo su -c 'echo "app-text/texlive-core xetex" > /etc/portage/package.use/texlive'
        sudo su -c 'echo "dev-libs/zziplib sdl" > /etc/portage/package.use/zziblib'

The new logic tries to be smart enough to detect for missing files
and missing arguments. Yet, as Gentoo seems to require users to
manage those package.use files by hand, the logic isn't perfect:
users may still need to verify for conflicts on different use
files.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/365fe5e7d568da932dcffde65f48f2c1256cb773.1754992972.git.mchehab+huawei@kernel.org

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