allow packages to hook into code loading

[KristofferC]

This is so e.g. Pkg3 can install packages on demand:

[fredrikekre]

Notes from Pkg triage:

Prompting for package additionTop-level X:

• happens when doing import X at top-level if X is not in load path
• “Add package X?”
• add X
• resolve (implicit)Dev’d non-top-level package:
• happens when Y does import X without Y in its deps
• “Add package X to project Y?”
• activate Y
• add X
• deactivate
• resolve (edited)
  both cases need to have a package called X
  if there’s two packages called X they both need to ask which one you want

[DilumAluthge]

This is only for the REPL, right? Presumably, if you are running a script, then using Foo should still throw an error if the package Foo is not found.

[Keno]

I think a better implementation of this would be to throw a special error type that the REPL intercepts. That way the REPL (and any other frontend offering code evaluation) can be in charge of presenting the UI without needing to inject callbacks in the core loading code.

[IanButterworth]

I've given @Keno's suggestion a go over here #39026

[oxinabox]

Can this now be closed?