feat(external): Implement tool to analyse external dependencies

Adds a tool that can analyse dependencies that were not originally
meant to be built with buildGo.nix and return information that can be
used to construct appropriate Nix dependencies.

The tool will return information about package-local and foreign
dependencies separately to let Nix determine whether all required
dependencies are provided and to correctly link together sub-packages.

To avoid listing standard library imports in the dependencies, a list
of all packages in the standard library is generated statically to
allow for those to be filtered out during the analysis.

This tool is still work-in-progress.

external/default.nix

@@ -0,0 +1,29 @@
+# Copyright 2019 Google LLC.
+# SPDX-License-Identifier: Apache-2.0
+{ runCommand, go, jq, ripgrep, program }:
+
+let
+  # Collect all non-vendored dependencies from the Go standard library
+  # into a file that can be used to filter them out when processing
+  # dependencies.
+  stdlibPackages = runCommand "stdlib-pkgs.json" {} ''
+    export GOPATH=/dev/null
+    ${go}/bin/go list all | \
+      ${ripgrep}/bin/rg -v 'vendor' | \
+      ${jq}/bin/jq -R '.' | \
+      ${jq}/bin/jq -c -s 'map({key: ., value: true}) | from_entries' \
+      > $out
+  '';
+
+  analyser = program {
+    name = "analyser";
+
+    srcs = [
+      ./main.go
+    ];
+
+    x_defs = {
+      "main.stdlibList" = "${stdlibPackages}";
+    };
+  };
+in analyser