lib: init flake subset

lib/flake.nix

@@ -0,0 +1,27 @@
+# No, this is not a flake of the library set, it is a library subset for
+# flake-related shtick. This should be used VERY RARELY in most parts of the
+# configuration because they are set up to be usable both in flakes- and
+# non-flakes-enabled environment.
+#
+# Take note it has a very strict design constraint of not relying on the
+# `inputs` attribute of the flake output. Instead, we're relying on the
+# builtins and the flake lockfile for this.
+#
+# Because Nix under the hood is a bit of a mess especially relating with
+# flakes, we'll have to thoroughly document which versions this is working. So
+# far, it is working on flake format version 7. AAAAAND also because apparently
+# it requires flakes to be enabled to use `builtins.fetchTree`, this is pretty
+# much a flakes-only subset.
+#
+# If anything else is pretty much in despair, we could always ste- I mean...
+# copy edolstra's flake-compat implementation.
+rec {
+  importFlakeMetadata = flakeLockfile:
+    builtins.fromJSON (builtins.readFile flakeLockfile);
+
+  fetchTree = metadata: inputName:
+    builtins.fetchTree metadata.nodes.${inputName}.locked;
+
+  fetchInput = metadata: inputName:
+    (fetchTree metadata inputName).outPath;
+}