Support unused dependencies

Cargo stashes unused patches into a special section of the Cargo.lock
file:

```
[[patch.unused]]
name = "uuid"
version = "1.4.1"
source = "git+https://github.com/uuid-rs/uuid#50f70278de02c106650b8d6deb325dd59b5f2a24"
```

... a section that we, currently, don't read.

Ignoring that section causes the build to fail because even if the patch
is unused, when the source is unavailable, Cargo tries to fetch it
(which, understandably, is not possible inside the sandbox).

This commit extends our logic so that we download both the "used" and
"unused" dependencies.

Closes #308.

lib.nix

@@ -99,7 +99,14 @@ rec
       } // (lib.optionalAttrs (! isNull branch) { inherit branch; })
         // (lib.optionalAttrs (! isNull tag) { inherit tag; })
         // (lib.optionalAttrs (! isNull rev) { inherit rev; });
-      packageLocks = builtins.map parseLock (lib.filter query cargolock.package);
+
+      usedPackageLocks =
+        builtins.map parseLock (lib.filter query cargolock.package);
+
+      unusedPackageLocks =
+        builtins.map parseLock (lib.filter query ((cargolock.patch or []).unused or []));
+
+      packageLocks = usedPackageLocks ++ unusedPackageLocks;
 
       mkFetch = lock: {
         key = lock.rev or lock.tag or lock.branch or lock.revision

test/fast/default.nix

@@ -16,6 +16,7 @@ args: {
   simple-dep = import ./simple-dep args;
   simple-dep-patched = import ./simple-dep-patched args;
   symlinks = import ./symlinks args;
+  unused-patch = import ./unused-patch args;
   workspace = import ./workspace args;
   workspace-build-rs = import ./workspace-build-rs args;
   workspace-patched = import ./workspace-patched args;

test/fast/unused-patch/default.nix

@@ -0,0 +1,15 @@
+{ naersk, pkgs, ... }:
+let
+  app = naersk.buildPackage {
+    src = ./fixtures;
+  };
+
+in
+if builtins.compareVersions pkgs.lib.version "22.11" <= 0 then
+  # Executing this test requires nixpkgs > 22.11 due to changes to the TOML
+  # serialization function.
+  #
+  # See `writeTOML` in this repository for more details.
+  true
+else
+  app

test/fast/unused-patch/fixtures/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "app"
+version = "0.1.0"
+edition = "2018"
+
+[patch.crates-io]
+uuid = { git = "https://github.com/uuid-rs/uuid" }

test/fast/unused-patch/fixtures/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}