lowering: Don't closure-convert in import or using

[mlechu]

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.

[topolarity]

Thanks @mlechu !