Handle default layer with custom name

https://unifiedfontobject.org/versions/ufo3/layercontents.plist/ says:

> # public.default
>
> This layer name indicates that the default layer has no user defined name. It must not be used as the layer name for any directory other than the default glyphs directory.

GlyphLib assumes the default layer has no user defined name but that may not be the case.
https://github.com/googlefonts/glyphsLib/blob/638cbddd00b2334410bab914d932bf673d6342ca/Lib/glyphsLib/builder/layers.py#L74
Instead it should check that the layer is `font.layers.defaultLayer` or that its name is `font.layers.defaultLayer.name`.

Start in Lib/glyphsLib/builder/layers.py at line 74 and compare the current default-layer check with font.layers.defaultLayer and its name. Verify that conversion handles a default layer with a custom name while preserving the restriction on non-default layers, then run the relevant test suite.

Written by the indexing model from the issue text.