Reimplement the `ModulePath` generation logic by only adding the
`/lib/xorg/modules` subpath for each module, in the specified order.
In particular, hardware-specific drivers are listed *before*
`xorgserver`, which fixes https://github.com/NixOS/nixpkgs/issues/299684.
This also keeps the list reproducible, as wanted by https://github.com/NixOS/nixpkgs/pull/230186.
I have confirmed that X is able to find `.so` files recursively within
the `ModulePath`, so that there is no need to include subdirectories of
`/lib/xorg/modules`. Furthermore, I don't expect there to be a need to
include directories *outside* of `/lib/xorg/modules`, as the default
`ModulePath` on standard distributions is `/usr/lib/xorg/modules`.
(see https://www.x.org/releases/current/doc/man/man5/xorg.conf.5.xhtml#heading4)
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