confusing error when Pkg is not imported but REPL mode has been used

Although Pkg is loaded, it's not accessible from Main here, so the hint `a global variable of this name also exists in Pkg.` seems confusing here:

```julia-repl
(@v1.12) pkg> instantiate

julia> Pkg
ERROR: UndefVarError: `Pkg` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: a global variable of this name also exists in Pkg.
```