node_modules/.bin/ is expected to be (symlink to) a directory rather
than a regular file, so we check for that.
This seems to work for pre-existing directories as well as those
we link to the store (managed & unmanaged in the hooks terminology).
I am not entirely sure whether the original check was simply a typo
or intentional for some use-cases.
`importNpmLock.buildNodeModules` returns a derivation with a pre-built `node_modules` directory, as imported by `importNpmLock`.
This is to be used together with `importNpmLock.hooks.linkNodeModulesHook` to facilitate `nix-shell`/`nix develop` based development workflows:
```nix
pkgs.mkShell {
packages = [
importNpmLock.hooks.linkNodeModulesHook
nodejs
];
npmDeps = importNpmLock.buildNodeModules {
npmRoot = ./.;
inherit nodejs;
};
}
```
will create a development shell where a `node_modules` directory is created & packages symlinked to the Nix store when activated.
This code is adapted from https://github.com/adisbladis/buildNodeModules
