Switch libunwind to overlay convention

.orchestra/config/components.yml

 #@ load("@ytt:template", "template")
 #@ load("@ytt:data", "data")
 
-#@ load("/components/libunwind.lib.yml", "libunwind_component")
 #@ load("/components/llvm.lib.yml", "llvm_component")
 #@ load("/components/llvm_documentation.lib.yml", "llvm_documentation_component")
 #@ load("/components/llvmcpy.lib.yml", "llvmcpy_component")
@@ -49,7 +48,6 @@
 
 #@ load("/global_options.lib.yml", "options")
 components:
-  libunwind: #@ libunwind_component
   llvm: #@ llvm_component
   llvm-documentation: #@ llvm_documentation_component
   llvmcpy: #@ llvmcpy_component

.orchestra/config/components/libunwind.lib.yml → .orchestra/config/components/libunwind.yml

+#@ load("@ytt:overlay", "overlay")
 #@ load("/lib/create_component.lib.yml", "single_build_component")
 
 #@ load("/global_options.lib.yml", "options")
@@ -31,6 +32,8 @@ dependencies:
   - toolchain/host/gcc
 #@ end
 
+#@overlay/match by=overlay.all, expects=1
+#@overlay/match-child-defaults missing_ok=True
 ---
-
-#@ libunwind_component = single_build_component(**_libunwind_args())
+components:
+  libunwind: #@ single_build_component(**_libunwind_args())