Link Semibold Regular/Italic faces into one (legacy) family name

[alerque]

Somewhere between v6.11 and v6.12, the Serif Semibold became two families. The italic style is showing up with the family name as Libertinus Serif Semibold Italic and subfamily Regular instead of both fonts being Libertinus Serif Semibold, one with one Subfamily Regular and one Italic.

The snafu with font names in #390 brought this up again. I was so worried about getting past the broken release to working v6.12/v7.000 parity that I didn't look into it further, but this could definitely use fixing as well.

Tangentially related would be dropping the legacy support altogether, see #392. But as long as we have it, it might as well be working properly.

From Khaled's comment:

Family:              Libertinus Serif Semibold
Subfamily:           Italic
Full name:           Libertinus Serif Semibold Italic
PostScript name:     LibertinusSerif-SemiboldItalic

This is what I expect, and it was that way in 6.11, no idea what broke it in 6.12.

[alerque]

Somewhat unsurprisingly I guess, this issue bisects to 33bce0e6 as the first bad commit.

[alerque]

I think the offending code is going to turn out to be this monkey business in ufo2ft:

https://github.com/googlefonts/ufo2ft/blob/b796cb7d225768645fbd47101424cc298de8fdc9/Lib/ufo2ft/fontInfoData.py#L70-L76

[khaledhosny]

You need to check the generated UFOs, quite possibly they might have the wrong names set.

[kenmcd]

Tangentially related would be dropping the legacy support altogether, see #392. But as long as we have it, it might as well be working properly.

If you drop this the fonts will not work properly in LibreOffice on Windows.

[alerque]

@kenmcd Thanks, but I already came to that conclusion. Hence why the related tracking issue #392 was closed, see comments there.