fix #23978, deprecation message for Base.Test

[JeffBezanson]

The binding deprecation mechanism is probably better for this case. After:

julia> using Base.Test
WARNING: Base.Test is deprecated, run `using Test` instead.
  likely near no file:0

julia> using Test

julia> @test true
Test Passed

To do this, I added a rule that if a deprecated binding has nothing as its value, then it's not actually imported, so it won't conflict with importing the replacement. I suppose we can revisit that if we ever need to deprecate a variable whose correct value is actually nothing...

[KristofferC]

Will @test true work after only using Base.Test?

[yuyichao]

It already doesn't.

[KristofferC]

I know. Hence "will".

[JeffBezanson]

No, it won't. I don't believe any deprecations automatically load code for you.

[KristofferC]

Ok, my thought was that since this breaks 100% of packages, perhaps it would be worth to add a special case for a more smooth transition.

[martinholters]

Most deprecations print a warning and do what they used to do. This one only prints a warning and does nothing else, which seems radically less useful. While for modules moved to an external package automatic loading in the deprecation looks dangerous indeed, in this case it does make sense IMHO.

[JeffBezanson]

Most deprecations print a warning and do what they used to do.

Though none of the deprecations for moved code do that --- they all throw errors. But it's not too hard to add special cases for using these modules.

[JeffBezanson]

Ok, now using or import of a Base.X package moved to stdlib will load it automatically.