build: use unified dependency expressions instead of weird fields

Instead of "deps", "deps_neg", and "deps_any" fields, just have a single
"deps" field, which changes from an array to a string. The string is now
an expression, which can contain the operators &&, ||, !, and allows
grouping with ( ).

It's probably overkill. If it gets a maintenance burden, we can switch
to specifiying the dep expressions as ASTs (or maybe eval()-able Python
expressions), and we could simplify the code that determines the reason
why a dependency is not fulfilled. The latter involves a complicated
conversion of the expression AST to DNF.

The parser is actually pretty simple, and pretty much follows:
https://en.wikipedia.org/wiki/Shunting_yard_algorithm

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