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`!
Use `mkTestDerivationFromNixvimModule` instead of `mkTestDerivation`,
allowing "proper" modules to be used instead of plain attr configs.
This is useful for more complex tests that wish to use `config` or
`options` arguments, e.g:
```nix
{config, options, ...}: {
/* some cool test */
}
```
To allow `tests.dontRun` to be defined on such a test, the module is
allowed to be nested as `module`, e.g:
```nix
{
tests.dontRun = true;
module = {config, options, ...}: {
/* a disabled test */
};
}
```
Also ended up doing some general cleanup, removing an unused function,
etc.
