Stricter checking of brackets for conditional values in the new configspace format

[mfeurer]

The current code suggests that something like

x | y > 5 in {1, 2, 3 && z in 4, 5, 6}

which should be forbidden according to the specs.

[eddiebergman]

I'd rather we freeze pcs support as is and just encourage json or yaml formats which do not require a custom string parser.