add compile_data argument for data required at compile time (#516)

Using the existing data argument for build-time data dependencies causes the files to also show up in the runfiles. This can be seen when using rules_docker's rust_image() for example - the files are included in the image twice; once in the binary and once in their original form.

If this is accepted I'll send through a PR to cargo-raze to expose this for third-party crates similar to data_attr.

Closes #222

cargo/cargo_build_script.bzl

@@ -113,7 +113,8 @@ def _build_script_impl(ctx):
     env.update(expand_locations(
         ctx,
         ctx.attr.build_script_env,
-        getattr(ctx.attr, "data", []),
+        getattr(ctx.attr, "data", []) +
+        getattr(ctx.attr, "compile_data", []),
     ))
 
     tools = depset(