How to make dub recursively build subPackages?

[Vijay Nayar]

Greetings folks,

In my project, I have a parent package with several sub-packages, 
each of which builds into either a library or an executable.

I first started observing odd problems when I was running `dub 
build`, it would complain about different versions of vibe-d 
present, and it suggested running `dub upgrade`. After doing 
this, I noticed that most subPackages were not actually being 
upgraded.

The only thing I have found thus far is to manually run each 
subPackage one at a time, e.g. `dub :proto; dub :common; ...`.

Is it possible to get `dub upgrade` to recursively work on all 
sub-packages?

My parent package dub.sdl file:
```
name "funnel"
description "An in-memory queryable database for processing 
extreme loads of current data."
authors "Vijay Nayar"
copyright "Copyright © 2019, Vijay Nayar"
license "proprietary"
targetType "none"
targetPath "target"
dependency "funnel:proto" version=">=0.0.0"
dependency "funnel:spout" version=">=0.0.0"
dependency "funnel:stem" version=">=0.0.0"
dependency "funnel:mouth" version=">=0.0.0"
dependency "funnel:common" version=">=0.0.0"
subPackage "./common"
subPackage "./proto"
subPackage "./mouth"
subPackage "./stem"
subPackage "./spout"
```

Each subPackage is structured in the same way, for example, the 
common subPackage:
```
authors "Vijay Nayar"
copyright "Copyright © 2019, Vijay Nayar"
description "Common logic between the mouth and spout components."
license "proprietary"
name "common"
targetType "library"
targetPath "target"
dependency "funnel:proto" version="*"
dependency "poodinis" version="~>8.0.3"
dependency "vibe-d" version="~>0.9.4"
```

I mostly followed the dub documentation in setting up my project. 
https://dub.pm/package-format-sdl.html