Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
Sometimes plugins doesn't follow the form
```
addSbtPlugin("${plugin.org}" % "${plugin.artifact}" % "${plugin.version}")
```
for instance
```
addDependencyTreePlugin
```
This commit allow free form plugin dependency.
