Simplifications to expand definitions (#6487)

Simplifications to expand definitions (#6487)

This removes the expandOnly flag from expandDefinitions.

The use of expandOnly = true is equivalent to applying top-level substitutions only, which should be done explicitly via Env::getTopLevelSubstitutions. It updates the trust substitutions utility to distinguish apply vs applyTrusted for convenience for this purpose.

This also breaks several dependencies in e.g. expand definitions module.