lowering: Don't closure-convert in `import` or `using` (#57774)
Fixes #57702. We're calling cl-convert- on `using` and `import`
statements when we shouldn't, so if there's a nearby local that gets
boxed (recursive function definition in this case), and the local shares
a name with something in an import statement, we get a box access where
we want a raw symbol.
Before:
```
julia> let; let; import SHA: R; end; let; R(x...) = R(x); end; end
ERROR: TypeError: in import, expected Symbol, got a value of type Expr
Stacktrace:
[1] top-level scope
@ REPL[1]:1
```
After:
```
julia> let; let; import SHA: R; end; let; R(x...) = R(x); end; end
(::var"#R#R##0") (generic function with 1 method)
```
Previously, symbols in `import`/`using` statements would be wrapped with
`outerref`, which cl-convert- wouldn't peek into. This protected us from
this problem in 1.11.
