Matching on sub-family by font name doesn't work on Linux

If a font has separate base font-name and preferred font name, the fontconfig fontdatabase will register a font face under the preferred font name, and set the base font-name as an alias. When the base name is a sub-family however that means when that sub-family is requested we will incorrectly look at the whole family of face and not just the sub-family.

For instance looking for family "MyFont Light", will redirect to "MyFont" and return any face from there.

This works on Windows.