* Leave importing to nixpkgs module implentation. Provide a path
instead; resolves#136.
* Allow profiles which are not lambdas but simple attribute sets,
relaxing the constraints a bit.
* Update profile README.md
* defaultImports -> mkProfileAttrs: allow importing subprofiles even
if parent directory does not contain a default.nix.
This core will be used as the new base for the reuse branch, which is
essentiall the same as the existing template branch, except that it is
rebased from this core, simplifying future rebasing.
* Import attrs of profiles automatically with `defaultImport`.
* Refactor profiles to ensure all are functions returning a module.
* Add a suites.nix with collections of profiles.
* Add suites as `specialArgs` to modules.
* Add suite import to NixOS host.
Keep the README.md lean and use DOC.md for more detailed explanations. While
revising user documentation, a default `nixos` user profile was created, which
serves as a goood base example.
