[antlir2] Support root-relative dst_path in feature

Summary: It's possible for `dst_path` to already be root-relative (particularly when compiling one feature from another feature), so handle that in the branching.

Test Plan: CI + used in above diffs in stack

Reviewed By: sergeyfd

Differential Revision: D47532604

fbshipit-source-id: fddf77ee2c03c36ad2c4ff6003c4f370ed9b57f7

antlir/antlir2/antlir2_compile/src/lib.rs

@@ -202,6 +202,8 @@ impl CompilerContext {
     {
         if !path.as_ref().is_absolute() {
             self.root.join(path)
+        } else if path.as_ref().starts_with(&self.root) {
+            path.as_ref().to_path_buf()
         } else {
             self.root
                 .join(path.as_ref().strip_prefix("/").expect("infallible"))