refactor(tvix): move castore into tvix-castore crate

This splits the pure content-addressed layers from tvix-store into a
`castore` crate, and only leaves PathInfo related things, as well as the
CLI entrypoint in the tvix-store crate.

Notable changes:
 - `fixtures` and `utils` had to be moved out of the `test` cfg, so they
   can be imported from tvix-store.
 - Some ad-hoc fixtures in the test were moved to proper fixtures in the
   same step.
 - The protos are now created by a (more static) recipe in the protos/
   directory.

The (now two) golang targets are commented out, as it's not possible to
update them properly in the same CL. This will be done by a followup CL
once this is merged (and whitby deployed)

Bug: https://b.tvl.fyi/issues/301

Change-Id: I8d675d4bf1fb697eb7d479747c1b1e3635718107
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9370
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/proto/default.nix

@@ -1,9 +1,15 @@
-# Build protocol buffer definitions to ensure that protos are valid in
-# CI. Note that the output of this build target is not actually used
-# anywhere, it just functions as a CI check for now.
-{ pkgs, ... }:
+# Target containing just the proto files used in tvix
 
-pkgs.runCommand "tvix-cc-proto" { } ''
-  mkdir $out
-  ${pkgs.protobuf}/bin/protoc -I ${./.} evaluator.proto --cpp_out=$out
-''
+{ depot, lib, ... }:
+
+depot.nix.sparseTree {
+  name = "tvix-protos";
+  root = depot.path.origSrc;
+  paths = [
+    ../castore/protos/castore.proto
+    ../castore/protos/rpc_blobstore.proto
+    ../castore/protos/rpc_directory.proto
+    ../store/protos/pathinfo.proto
+    ../store/protos/rpc_pathinfo.proto
+  ];
+}