Add @usingif

[timholy]

Useful? This might help with #2025.

[pao]

I think this makes more sense in Pkg than in Meta; it's not really a metaprogramming tool.

[mlubin]

+1 bump

[quinnj]

I ran into needing this the other day. +1

[timholy]

Update: I think the "cleaner way" was wrong, so I don't currently have a cleaner approach.

I've since nuked the branch this was based on, but I can clean it up and push if this is something mergeable. The suggestions I've gotten:

• Make the status an optional second argument, rather than the first argument. I suspect you'd almost always want to know whether the module was loaded, so I am a bit skeptical this would be used much. Unless there's a better way to test whether it's been loaded? names is not exactly the most efficient approach, but we could go that way.
• I can put this into Pkg rather than Meta

If I don't hear objections in the next week or so, I'll make these changes and just push it.