This renames our `firmwareLinuxNonfree` package to `linux-firmware`.
There is prior art for this in multiple other distros[1][2][3].
Besides making the package more discoverable by those searching for the
usual name, this also brings it in-line with the `kebab-case` we
normally see in `nixpkgs` pnames, and removes the `Nonfree` information
from the name, which I consider redundant given it's present in
`meta.license`.
The corresponding alias has been added, so this shouldn't break
anything.
[1]: https://archlinux.org/packages/core/any/linux-firmware/
[2]: https://src.fedoraproject.org/rpms/linux-firmware
[3]: https://packages.gentoo.org/packages/sys-kernel/linux-firmware
