feat(ops/modules): Provide some modules to all nixoses

For modules that are gated behind a mkEnableOption, it's reasonable to
just provide them to all Depot-built nixos systems without requiring
people to explicitly import them. This defines a special module called
`default-imports.nix` which imports these modules (currently just
tvl-cache.nix and automatic-gc.nix, as I'm being rather conservative
adding things here to avoid breaking anyone's system), then provides
that module as one of the `modules` passed at the top-level
nixos/eval-config invocation.

Change-Id: I3be299ab10ae4c451ef11c514edb3c89318a2278
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4345
Tested-by: BuildkiteCI
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

ops/modules/default-imports.nix

@@ -0,0 +1,13 @@
+{ depot, ... }:
+
+# Default set of modules that are imported in all Depot nixos systems
+#
+# All modules here should be properly gated behind a `lib.mkEnableOption` with a
+# `lib.mkIf` for the config.
+
+{
+  imports = [
+    ./automatic-gc.nix
+    ./tvl-cache.nix
+  ];
+}