Promote standalone test cases to packages

This is a prerequisite for removing `b.anonymousDependency()`, but
having compiler tests dogfood package management might be a good idea
in general.

build.zig.zon

@@ -0,0 +1,12 @@
+// The Zig compiler is not intended to be consumed as a package.
+// The sole purpose of this manifest file is to test the compiler.
+.{
+    .name = "zig",
+    .version = "0.0.0",
+    .dependencies = .{
+        .standalone_test_cases = .{
+            .path = "test/standalone",
+        },
+    },
+    .paths = .{""},
+}