flake.nix : rewrite (#4605)

* flake.lock: update to hotfix CUDA::cuda_driver

Required to support https://github.com/ggerganov/llama.cpp/pull/4606

* flake.nix: rewrite

1. Split into separate files per output.

2. Added overlays, so that this flake can be integrated into others.
   The names in the overlay are `llama-cpp`, `llama-cpp-opencl`,
   `llama-cpp-cuda`, and `llama-cpp-rocm` so that they fit into the
   broader set of Nix packages from [nixpkgs](https://github.com/nixos/nixpkgs).

3. Use [callPackage](https://summer.nixos.org/blog/callpackage-a-tool-for-the-lazy/)
   rather than `with pkgs;` so that there's dependency injection rather
   than dependency lookup.

4. Add a description and meta information for each package.
   The description includes a bit about what's trying to accelerate each one.

5. Use specific CUDA packages instead of cudatoolkit on the advice of SomeoneSerge.

6. Format with `serokell/nixfmt` for a consistent style.

7. Update `flake.lock` with the latest goods.

* flake.nix: use finalPackage instead of passing it manually

* nix: unclutter darwin support

* nix: pass most darwin frameworks unconditionally

...for simplicity

* *.nix: nixfmt

nix shell github:piegamesde/nixfmt/rfc101-style --command \
    nixfmt flake.nix .devops/nix/*.nix

* flake.nix: add maintainers

* nix: move meta down to follow Nixpkgs style more closely

* nix: add missing meta attributes

nix: clarify the interpretation of meta.maintainers

nix: clarify the meaning of "broken" and "badPlatforms"

nix: passthru: expose the use* flags for inspection

E.g.:

```
❯ nix eval .#cuda.useCuda
true
```

* flake.nix: avoid re-evaluating nixpkgs too many times

* flake.nix: use flake-parts

* nix: migrate to pname+version

* flake.nix: overlay: expose both the namespace and the default attribute

* ci: add the (Nix) flakestry workflow

* nix: cmakeFlags: explicit OFF bools

* nix: cuda: reduce runtime closure

* nix: fewer rebuilds

* nix: respect config.cudaCapabilities

* nix: add the impure driver's location to the DT_RUNPATHs

* nix: clean sources more thoroughly

...this way outPaths change less frequently,
and so there are fewer rebuilds

* nix: explicit mpi support

* nix: explicit jetson support

* flake.nix: darwin: only expose the default

---------

Co-authored-by: Someone Serge <sergei.kozlukov@aalto.fi>

flake.lock

@@ -1,30 +1,30 @@
 {
   "nodes": {
-    "flake-utils": {
+    "flake-parts": {
       "inputs": {
-        "systems": "systems"
+        "nixpkgs-lib": "nixpkgs-lib"
       },
       "locked": {
-        "lastModified": 1694529238,
-        "narHash": "sha256-zsNZZGTGnMOf9YpHKJqMSsa0dXbfmxeoJ7xHlrt+xmY=",
-        "owner": "numtide",
-        "repo": "flake-utils",
-        "rev": "ff7b65b44d01cf9ba6a71320833626af21126384",
+        "lastModified": 1701473968,
+        "narHash": "sha256-YcVE5emp1qQ8ieHUnxt1wCZCC3ZfAS+SRRWZ2TMda7E=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "34fed993f1674c8d06d58b37ce1e0fe5eebcb9f5",
         "type": "github"
       },
       "original": {
-        "owner": "numtide",
-        "repo": "flake-utils",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
         "type": "github"
       }
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1698318101,
-        "narHash": "sha256-gUihHt3yPD7bVqg+k/UVHgngyaJ3DMEBchbymBMvK1E=",
+        "lastModified": 1703559957,
+        "narHash": "sha256-x9PUuMEPGUOMB51zNxrDr2QoHbYWlCS2xhFedm9MC5Q=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "63678e9f3d3afecfeafa0acead6239cdb447574c",
+        "rev": "75dd68c36f458c6593c5bbb48abfd3e59bfed380",
         "type": "github"
       },
       "original": {
@@ -34,26 +34,29 @@
         "type": "github"
       }
     },
-    "root": {
-      "inputs": {
-        "flake-utils": "flake-utils",
-        "nixpkgs": "nixpkgs"
-      }
-    },
-    "systems": {
+    "nixpkgs-lib": {
       "locked": {
-        "lastModified": 1681028828,
-        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
-        "owner": "nix-systems",
-        "repo": "default",
-        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "dir": "lib",
+        "lastModified": 1701253981,
+        "narHash": "sha256-ztaDIyZ7HrTAfEEUt9AtTDNoCYxUdSd6NrRHaYOIxtk=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e92039b55bcd58469325ded85d4f58dd5a4eaf58",
         "type": "github"
       },
       "original": {
-        "owner": "nix-systems",
-        "repo": "default",
+        "dir": "lib",
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
         "type": "github"
       }
+    },
+    "root": {
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "nixpkgs": "nixpkgs"
+      }
     }
   },
   "root": "root",