Fix cfg generation

[kennykerr]

This update fixes a collection of cfg generation bugs.

• the deprecated feature was over/under applied
• feature dependencies weren't being properly folded
• nested cfg declarations were redundantly declaring required features

This fixes #3425 and builds on #3430. The process of converting dependencies into cfg features is still somewhat tortured but at least now I believe we're a lot closer to quality cfg generation and I can continue to improve the internals without having to deal with a huge diff for code generation validation.