wd_rust_crate improvements (#5180)

- proper include prefix
- @expand manual target to debug proc macros (if there are any)

Author: mikea

build/wd_rust_crate.bzl

@@ -1,4 +1,4 @@
-load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test")
+load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test", "rust_unpretty")
 
 def rust_cxx_bridge(
         name,
@@ -87,13 +87,15 @@ def wd_rust_crate(
             "@workerd-cxx//:cxx",
         ]
 
+    include_prefix = "workerd/" + native.package_name().removeprefix("src/")
+
     hdrs = native.glob(["**/*.h"], allow_empty = True)
     for bridge_src in cxx_bridge_srcs:
         rust_cxx_bridge(
             name = bridge_src + "@cxx",
             src = bridge_src,
             hdrs = hdrs,
-            include_prefix = "workerd/rust/" + name,
+            include_prefix = include_prefix,
             strip_include_prefix = "",
             # Not applying visibility here – if you import the cxxbridge header, you will likely
             # also need the rust library itself to avoid linker errors.
@@ -139,3 +141,10 @@ def wd_rust_crate(
             "//conditions:default": 1,
         }),
     )
+
+    if len(proc_macro_deps) + len(cxx_bridge_srcs) > 0:
+        rust_unpretty(
+            name = name + "@expand",
+            deps = [":" + name],
+            tags = ["manual", "off-by-default"],
+        )