glyphsLib mangles the master name for ExtraLight from Glyphs 2

Trying to use glyphsLib to do some things and the ExtraLight master and instances get renamed to "Light Extra" if format is Glyphs 2.

```
import glyphsLib
font = glyphsLib.GSFont('glyphsLibTest.glyphs')
font.save('glyphsLibTest-saved.glyphs')
```
And ExtraLight is now "Light Extra"

[glyphsLibTest.glyphs.zip](https://github.com/googlefonts/glyphsLib/files/10938052/glyphsLibTest.glyphs.zip)

Reproduce the issue with the provided glyphsLibTest.glyphs archive using glyphsLib.GSFont to load and save the Glyphs 2 file. Trace the Glyphs 2 conversion path responsible for the master and instance names, and verify that ExtraLight remains unchanged after saving.

Written by the indexing model from the issue text.