Ensure that collations that use the import mechanism instead of alias are properly deduplicated

[hsivonen]

#1965 is about the CLDR collation alias mechanism. Some collations are logically aliases but are implemented as duplicated data via import.

Ensure that these work and the data is deduplicated: gl: es bs: hr bs-Cyrl: sr fa-AF: ps sr-Latn: hr

[hsivonen]

CC @sffc

[sffc]

Now that we support custom parents for collation data, this is something we can fix if necessary. Note that ICU4C already performs this mapping in the icuexportdata.