name the candidate's real requirement in the no-versions diagnostic

[notatallshaw]

When every version of a package is rejected because it needs a version of a dependency that conflicts with one the resolver has already decided, the no-versions diagnostic printed requires bar != 1.0. That reads as though any bar other than 1.0 would satisfy the candidate, which points away from the real conflict: the candidate actually needs a specific range (for example bar==2.0) that happens to exclude the decided 1.0.

The range-block and root-block paths already print requires X in <range> but solution has it in <...>. The decision-block path now does the same, reading the declared dep range out of the pending_decision_dep_ranges union the look-ahead already keeps, so it prints requires bar in [2.0, 2.0] but solution has it in [1.0, 1.0].

That accumulator arrived on main with #590 for blocker membership widening. This PR originally added a second one of its own; that half is dropped, and what is left is the diagnostic.

Stacked on #472, so both ranges render through format_range rather than the internal debug repr.

[read-the-docs-community]

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