Equations picks constructor over local variable when resolving names

The following code fails on Coq 8.20 and Equations 1.3.1.

```coq
Equations foo (I : Type) : bool :=
  foo I := true.
```
The error message is `Unable to build a covering for: foo I In context:   I : Type`.

Renaming `I` to, *eg.*, `J` solves the problem but it should not be necessary as `I` shadows the constructor for `True`.