Hint or helper at REPL for using/importing non-package modules

[BioTurboNick]

module Foo
    a = 5
    export a
end
using Foo
#=
ERROR: ArgumentError: Package Foo not found in current path:
- Run `import Pkg; Pkg.add("Foo")` to install the Foo package.
=#

I think the reasonable approach, as discussed with @StefanKarpinski and @DilumAluthge, would be:

• If package not present and not in registry but a module is defined in Main, hint that the user needs to have the dot in front: using .Foo
• If package is in registry and a module is defined in Main, offer to install package. Possibly also note that a module with the same name is defined in Main, and provide hint. Or install package and then provide hint after.
• If package is already added and a module is defined in Main, import the package and provide hint.

This could potentially be done extending the hooks created for #39026

[DilumAluthge]

@IanButterworth as the author of #39026, any thoughts?

[KristofferC]

If package not present and not in registry but a module is defined in Main, hint that the user needs to have the dot in front: using .Foo

This seems fine except it shouldn't be in Main but in the module the require call happens.

If package is already added and a module is defined in Main, import the package and provide hint.

I don't think this should happen. using Foo has a specific meaning and that is to load a package from the project. Imo the hint is enough.