Although since `icu_capi` uses weak dependency syntax, cargo vendor doesn't
recognize it. So this command will copy unnecessary crates. To avoid it, I
would like to use modified version of icu_capi.
And this has another issue. `icu_capi`'s C++ headers isn't compatible with
clang [*1]. So we need the workaround for it.
ICU4X 1.3 has another change for data provider with `icu_capi`.
From ICU4X 1.3, there are new `icu_*_data` crates to custom data file, instead
of `icu_testdata`. So we have to add each data crate if using `icu_capi`.
*1 https://github.com/llvm/llvm-project/issues/70162
Differential Revision: https://phabricator.services.mozilla.com/D192902
Makoto Kato