fix: Reimplement crate_supports_no_std syntactic heuristic

[Veykril]

This was never correctly tested which caused us to lose the correct implementation.

[ChayimFriedman2]

Is there a specific reason for not simply querying the def map? The approach in this PR is faster as it avoids computing the def map, but this doesn't matter where it's used.

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[Veykril]

The def map stores the no_stdness for the current cfg set. This function wants to know whether the crate can be no_std under some arbitrary cfg configuration