lib/extend-lib.nix returns a nixpkg's lib extended with our own
helpers.
This is exposed as `helpers.extendedLib`, but when evaluating our
modules it should be assigned to `specialArgs.lib`.
Outside of our modules you must still access our helpers via
`config.lib.nixvim` or `config.lib.nixvim.extendedLib`.
Within helpers' sub-sections, `lib` is the extended lib.
Let's simplify things by defining all modules in `./plugins`, `./modules`
and `./wrappers/modules`.
Instead of currying `pkgs` into a bootstrapping module, we can require
`defaultPkgs` be provided as a special arg.
This refactor allows us to completely remove `flake-modules/modules.nix`!
In our basic template we used to provide a check based on
`mkTestDerivationFromNvim`. The issue with this check (that is handled
correctly internally) is that some plugins _can't_ be used in the test
environment, for example image.nvim like in #1085.
This commit introduces a new function to generate such checks,
`mkTestDerivationFromNixvimModule`, that wraps a nixvim configuration
instead of a built nvim instance.
Then a configuration can rely on the newly added
`helpers.enableExceptInTests` attribute to disable parts of the
configuration depending if it is evaluated in tests or in a real final
configuration.
Resolves#1085
