Allow register_variant() based on a family created with register_font()

[petrbouchal]

Rationale: on GHA, where it is difficult to get fonts registered correctly (and perhaps elsewhere), the workaround is to register_font() from files inside an installed package. Registering variants via register_variant() base on a family created by register_font() is currently not possible, as the font does not appear in the output of system_fonts(). So a variant with different features can only be created by running the entire call to register_font() again with only the features param altered. That gets lengthy with lots of font weights/widths...

This may or may not be related to #68 - I guess that if "system" fonts can be found in other directories, then this whole workaround becomes unnecessary. But from the discussion under #58 I also understand that where fonts are searched for may well not be in systemfonts' power to influence...