there's something buggy about TDNR and name resolution

[mitchellwrosen]

This transcript shows odd behavior:

```ucm
scratch/main> builtins.merge

one.foo : Nat
one.foo = 17

two.foo : Text
two.foo = "blah"

scratch/main> add

one.foo : Nat
one.foo = 18

bar : Nat
bar = foo + foo

Here, we expect `foo` to resolve to the `one.foo` in the file, because although it could also refer to `two.foo` in the namespace, that wouldn't type check.

Yet the transcript fails, and with a confusing/wrong error message, too:

Loading changes detected in scratch.u.

I couldn't figure out what foo refers to here:

  5 | bar = foo + foo

The name foo is ambiguous. Its type should be: Nat

I found some terms in scope that have matching names and types. Maybe you meant one of these:

one.foo : Nat one.foo : Nat