Pkg.jl: Introduce `down` as opposite of `up`

[oxinabox]

I wish to test that I have set the lower bounds in my project.toml correctly.
I thus want to check that with the oldest package versions that can be compatibly installed,
my tests still pass.

So I want to do the oppoite of ]up

[fredrikekre]

This should probably just be flag to pkg> test rather than a pkg> down command.

[oxinabox]

No, because when debugging I want to instantiate the set of package versions I am testing on.
And then I can incrementally raise my lower bound, and run pkg> resolve and then test again.

If it is important to me to have the lowest lower bound I can,
I might fiddle around with the code to make it work on an older version,
and so I will want that older version loaded so I can do prototyping in the REPL etc.

and especially when your tests take a long time
retesting just the snippet of code that breaks is good.

[StefanKarpinski]

pkg> down seems reasonable to me although it might just be a shortcut for pkg> up --lower or something like that.

[oxinabox]

matching this
pkg> rand
or pkg> up --random

To get a random set of compatible versions.