Add ability to blacklist local dependencies in `iotile-build`

For testing purposes, it can be useful to ensure that no local dependencies will be used when running

```
iotile depends update
```

This could be accomplished by having a boolean flag that blacklists the `ComponentRegistryResolver` in `DependencyManager.update()`:

https://github.com/iotile/coretools/blob/master/iotilebuild/iotile/build/dev/depmanager.py#L173

The use case is testing that a component (prior to releasing it) builds correctly when run using only released components and does not secretly rely on a locally installed development mode component.