Pkg: provide hooks for Pkg.update() to packages

[staticfloat]

It would be neat if packages could register a callback for when the user does Pkg.update(). This would be a really nice solution for packages that might internally have some state that they could update (such as the formulae in the tap of my Homebrew.jl package).

As it stands, right now there is a Homebrew.update() function which isn't terrible, but as we get more and more packages, it would be nice to have Pkg.update() be able to apply to all the packages that might have such functions. I have a dummy deps/build.jl file which is just:

using Homebrew
Homebrew.update()

But this only gets triggered on an SHA bump of Homebrew.jl, which will likely happen far less often than I will add new formulae into homebrew-juliadeps.

[Keno]

Maybe we should allow packages to put an update.jl file into their deps directory.

[staticfloat]

That sounds good to me. Especially as it keeps things lightweight; we don't necessarily have to pull in the entire package for parsing just to update it.

[StefanKarpinski]

We should go ahead with this. Where does the call to deps/update.jl need to go?

[staticfloat]

I would do it just after a successful Pkg.update().

[staticfloat]

Is this getting put into 0.2? Or 0.3?

[StefanKarpinski]

I think it could probably go into 0.2.

[jiahao]

Marking as 0.2.x - bonus if it makes it to 0.2 release

[staticfloat]

I've marked this as 0.3, since it's a new feature.