In ICU-20472, we changed the way res_index files are generated, which resulted in some empty locales being listed that hadn't previously been listed. However, alias locales according to icu-locale-deprecates.xml were still being removed.
I would like to go one step further: all locale files present in the icu4c/source/data tree should be added to res_index.
This has the following benefits:
1. Simpler to reason about.
2. More consistent: empty locales according to CLDR data (parent locales in supplementalData) get treated the same as empty locales according to the ICU configuration file (alias locales).
3. Less work for Python icutools.databuilder: removes the need to open the XML file at configure time.
There is additional discussion in ICU-20490.