run build scripts in the crate folder instead of the workspace

Various crates expect to be able to access files relative to the
crate root, and we were giving them the parent directory (eg __main__)
instead. An example crate that fails to compile without this change
is blake3.

cargo/cargo_build_script.bzl

@@ -20,7 +20,7 @@ def _build_script_impl(ctx):
     dep_env_out = ctx.actions.declare_file(ctx.label.name + ".depenv")
     flags_out = ctx.actions.declare_file(ctx.label.name + ".flags")
     link_flags = ctx.actions.declare_file(ctx.label.name + ".linkflags")
-    manifest_dir = "%s.runfiles/%s" % (script.path, ctx.label.workspace_name or ctx.workspace_name)
+    manifest_dir = "%s.runfiles/%s/%s" % (script.path, ctx.label.workspace_name or ctx.workspace_name, ctx.label.package)
     compilation_mode_opt_level = get_compilation_mode_opts(ctx, toolchain).opt_level
 
     crate_name = ctx.attr.crate_name

examples/cargo/build_script.rs

@@ -19,7 +19,7 @@ fn main() {
     let bleh = env::var("CARGO_FEATURE_BLEH").unwrap();
     let some_env = env::var("SOME_ENV").unwrap();
     let out_dir = env::var("OUT_DIR").unwrap();
-    let data = std::fs::read("cargo/test.txt").unwrap();
+    let data = std::fs::read("test.txt").unwrap();
     assert!(!bleh.is_empty());
     assert_eq!(some_env, "42");
     println!(r#"cargo:rustc-env=FOO=BAR