* Add flake.variants
This allows flakes to easily include variations of the
project by with different project arguments.
Anything you can pass to `project.addModule` can be used.
For instance to include variants using ghc 9.2.4:
```
flake.variants.ghc924.compiler-nix-name = pkgs.lib.mkForce "ghc924";
```
Then use it with:
```
nix build .#ghc924:hello:exe:hello
```
This change also:
* Exposes add `combineFlakes` and `rawFlake` for more control over how flakes are combined.
* Includes package coverageReports to the `hydraJobs` of the flakes.
* Tries to make the materialisation concurrency test more reliable
This adds support for using nix flakes commands with hix. This is done by creating a hidden `.hix-flake` directory and passing the source in using `--override-input`.
It also includes a `hix init` command to add a `flake.nix` and `nix/hix.nix` configuration file.
Hamish Mackenzie