Add warning to Pkg.update() if updated modules were already imported

[stevengj]

If you do Pkg.update(), it should keep track of whether it updates any modules that were already imported into Main. If so, it should print a warning message telling the user that they will need to restart Julia if they want to use the updated modules.

Should be pretty easy to implement. (I would prefer that we don't print the warning unconditionally, e.g. if no imported modules were updated.)

See also #18150, cc @vtjnash.

[helgee]

Like this?

pkgsym = Symbol(pkg)
if isdefined(Main, pkgsym) && typeof(getfield(Main, pkgsym)) == Module
    warn("Package $pkg has been updated but it was already imported.")
    warn("Restart Julia to use the updated version.")
end

[stevengj]

@helgee, yes, something like that. Note that you'll want to call warn only once (you can pass it a multi-line string). You can use isa(x, Foo) rather than typeof(x) == Foo.