Enable debug info for dev profile build dependencies

Cargo now [turns off debug symbols for build dependencies by
default][1], but that doesn't allow us to reuse those built
dependencies when building a proc-macro.

Fixes #928

[1]: rust-lang/cargo#11252

top-crates/src/main.rs

@@ -32,6 +32,7 @@ struct TomlPackage {
 #[serde(rename_all = "kebab-case")]
 struct BuildOverride {
     codegen_units: u32,
+    debug: bool,
 }
 
 /// A profile section in a Cargo.toml file
@@ -71,12 +72,12 @@ fn main() {
             dev: Profile {
                 codegen_units: 1,
                 incremental: false,
-                build_override: BuildOverride { codegen_units: 1 },
+                build_override: BuildOverride { codegen_units: 1, debug: true },
             },
             release: Profile {
                 codegen_units: 1,
                 incremental: false,
-                build_override: BuildOverride { codegen_units: 1 },
+                build_override: BuildOverride { codegen_units: 1, debug: false },
             },
         },
         dependencies: dependencies.clone(),