Add support for using an already-evaluated hive

Note that the interface hasn't been finalized yet.
After this commit, the discovery order is:

1. flake.nix `colmenaHive` output (prepared hive)
2. flake.nix `colmena` output (raw hive)
3. hive.nix (raw hive)

src/nix/hive/assets.rs

@@ -78,7 +78,7 @@ impl Assets {
             }
             HivePath::Flake(_) => {
                 format!(
-                    "with builtins; let assets = getFlake \"{assets_flake_uri}\"; hive = assets.colmenaEval; in ",
+                    "with builtins; let assets = getFlake \"{assets_flake_uri}\"; hive = assets.processFlake; in ",
                     assets_flake_uri = self.assets_flake_uri.as_ref().expect("The assets flake must have been initialized"),
                 )
             }

src/nix/hive/flake.nix

@@ -6,10 +6,38 @@
   };
 
   outputs = { self, hive }: {
-    colmenaEval = import ./eval.nix {
-      rawFlake = hive;
-      colmenaOptions = import ./options.nix;
-      colmenaModules = import ./modules.nix;
-    };
+    processFlake = let
+      compatibleSchema = "v0";
+
+      # Evaluates a raw hive.
+      #
+      # This uses the `colmena` output.
+      evalHive = rawFlake: import ./eval.nix {
+        inherit rawFlake;
+        hermetic = true;
+        colmenaOptions = import ./options.nix;
+        colmenaModules = import ./modules.nix;
+      };
+
+      # Uses an already-evaluated hive.
+      #
+      # This uses the `colmenaHive` output.
+      checkPreparedHive = hiveOutput: 
+        if !(hiveOutput ? __schema) then
+          throw ''
+            The colmenaHive output does not contain a valid evaluated hive.
+
+            Hint: Use `colmena.lib.makeHive`.
+          ''
+        else if hiveOutput.__schema != compatibleSchema then
+          throw ''
+            The colmenaHive output (schema ${hiveOutput.__schema}) isn't compatible with this version of Colmena.
+
+            Hint: Use the same version of Colmena as in the Flake input.
+          ''
+        else hiveOutput;
+    in
+      if hive.outputs ? colmenaHive then checkPreparedHive hive.outputs.colmenaHive
+      else evalHive hive;
   };
 }

src/nix/hive/tests/makehive-flake/flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1624626397,
+        "narHash": "sha256-+h0ulo5//RqStx6g6MDqD9MzgmBfeZ1VYxwEaSmw/Zs=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e1f8852faac7638e88d5e8a5b9ee2a7568685e3f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

src/nix/hive/tests/makehive-flake/flake.nix

@@ -0,0 +1,29 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    colmena.url = "git+file://@repoPath@";
+  };
+  outputs = { nixpkgs, colmena, ... }: {
+    colmenaHive = colmena.lib.makeHive {
+      meta = {
+        nixpkgs = import nixpkgs {
+          system = "x86_64-linux";
+        };
+      };
+
+      host-a = { name, nodes, pkgs, ... }: {
+        boot.isContainer = true;
+        time.timeZone = nodes.host-b.config.time.timeZone;
+      };
+      host-b = {
+        deployment = {
+          targetHost = "somehost.tld";
+          targetPort = 1234;
+          targetUser = "luser";
+        };
+        boot.isContainer = true;
+        time.timeZone = "America/Los_Angeles";
+      };
+    };
+  };
+}

src/nix/hive/tests/mod.rs

@@ -3,13 +3,14 @@
 use super::*;
 
 use std::collections::HashSet;
+use std::fs;
 use std::hash::Hash;
 use std::io::Write;
 use std::iter::{FromIterator, Iterator};
 use std::ops::Deref;
 use std::path::PathBuf;
 
-use tempfile::NamedTempFile;
+use tempfile::{Builder as TempFileBuilder, NamedTempFile};
 use tokio_test::block_on;
 
 macro_rules! node {
@@ -179,6 +180,44 @@ fn test_parse_flake() {
     ));
 }
 
+#[test]
+fn test_parse_makehive_flake() {
+    // make a copy of the flake so we can edit the colmena input
+    let src_dir = PathBuf::from("./src/nix/hive/tests/makehive-flake");
+    let flake_dir = TempFileBuilder::new()
+        .prefix("makehive-flake-")
+        .tempdir()
+        .unwrap();
+
+    for entry in fs::read_dir(src_dir).unwrap() {
+        let entry = entry.unwrap();
+        if entry.file_type().unwrap().is_file() {
+            fs::copy(entry.path(), flake_dir.as_ref().join(entry.file_name())).unwrap();
+        }
+    }
+
+    let flake_nix = flake_dir.as_ref().join("flake.nix");
+    let patched_flake = fs::read_to_string(&flake_nix)
+        .unwrap()
+        .replace("@repoPath@", env!("CARGO_MANIFEST_DIR"));
+
+    fs::write(flake_nix, patched_flake).unwrap();
+
+    // run the test
+    let flake = block_on(Flake::from_dir(flake_dir)).unwrap();
+
+    let hive_path = HivePath::Flake(flake);
+    let mut hive = block_on(Hive::new(hive_path)).unwrap();
+
+    hive.set_show_trace(true);
+
+    let nodes = block_on(hive.deployment_info()).unwrap();
+    assert!(set_eq(
+        &["host-a", "host-b"],
+        &nodes.keys().map(NodeName::as_str).collect::<Vec<&str>>(),
+    ));
+}
+
 #[test]
 fn test_parse_node_references() {
     TempHive::valid(