I build: i18n: --with-intl=small-icu by default (English only). Avoid downloads.

* if `deps/icu` is there, use it
* in the `small-icu` default case, copy `deps/icu-small` to `deps/icu` before continuing
 * if the `deps/icu-small` dir doesn't exist, download full ICU source
* if you request `full-icu`, we still might have to download the source if needed.