refactor(3p/nix/libexpr): Move EvalState:* implementations out of parser.y

Several definitions of functions declared in eval.hh were previously
implemented in parser.y, this moves them over to parser.cc.

While this still isn't a reasonable place to keep them, the long-term
fix is more likely to be that eval.hh needs to be split up.

Before we get to that point however, this already gives us the ability
to use tooling with this code.

Change-Id: If06fb655325fe281564047ffab0a0a640428a0ee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1219
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/parser.hh

@@ -53,5 +53,6 @@ void addFormal(const Pos& pos, Formals* formals, const Formal& formal);
 Expr* stripIndentation(const Pos& pos, SymbolTable& symbols,
                        std::vector<Expr*>& es);
 
+Path resolveExprPath(Path path);
 
 }  // namespace nix