feat(kit): Export TVL kit as a josh workspace

This creates a josh workspace called `views/kit` which exports all
dependencies needed by external users of the TVL kit.

The intention of this is to have a single clonable "virtual repo"
which provides everything needed to run a system like TVL, but without
the projects that we develop *in* TVL.

Right now this is limited to the basic Nix code (readTree, pipeline
generation) and things required for Gerrit integration.

The workspace maps depot paths into slightly different (simpler)
locations inside the exported view, to avoid a nested, mostly empty
structure.

Note: This is not an "external API" or some such of depot, it is
simply a convenience for exporting some of our tooling.

Change-Id: Ied24aaef117fc2e0c188ec742ae7cd993a5babd3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4991
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

views/.skip-subtree

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+//views is not part of the depot build tree, see the README for more
+information.

views/README.md

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+depot views
+===========
+
+This folder contains external views of depot content, defined using
+josh workspaces. See the individual views for a description of their
+content and usage information.

views/kit/README.md

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+The TVL Kit
+===========
+
+This folder contains a publicly available version of the core TVL
+tooling, currently comprising of:
+
+* `buildkite`: TVL tooling for dynamically generating Buildkite
+  pipelines with Nix.
+* `buildGo`: Nix-based build system for Go.
+* `readTree`: Nix library to dynamically compute attribute trees
+  corresponding to the physical layout of a repository.
+* `besadii`: Configurable Gerrit/Buildkite integration hook.
+
+It can be accessed via git by cloning it as such:
+
+    git clone https://code.tvl.fyi/depot.git:workspace=views/kit.git tvl-kit
+
+If you are looking at this within the TVL depot, you can see the
+[josh][] configuration in `workspace.josh`. You will find the projects
+at slightly different paths within the depot.
+
+[josh]: https://github.com/josh-project/josh/

views/kit/default.nix

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+# Externally importable TVL depot stack. This is intended to be called
+# with a supplied package set, otherwise the package set currently in
+# use by the TVL depot will be used.
+#
+# For now, readTree is not used inside of this configuration to keep
+# it simple. Adding it may be useful if we set up test scaffolding
+# around the exported workspace.
+
+{ pkgs ? import ./nixpkgs { depotOverlays = false; }, ... }:
+
+pkgs.lib.fix(self: {
+  buildGo = import ./buildGo { inherit pkgs; };
+  buildkite = import ./buildkite { inherit pkgs; };
+  readTree = import ./readTree { };
+
+  besadii = import ./besadii {
+    depot.nix.buildGo = self.buildGo;
+  };
+})

views/kit/workspace.josh

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+::LICENSE
+besadii = :/ops/besadii
+:/nix:[
+    ::buildGo/
+    ::buildkite/
+    ::readTree/
+]
+nixpkgs = :/third_party/nixpkgs