[hooks] Assets only needed during development time

[dcharkes]

@liamappelbe brought up an interesting use case: CodeAssets which are only used in development time. For example something for running tests.

In pub we have dependencies and dev_dependencies, and dev_dependencies are not taken into consideration when a package is used as a dependency.

If an asset is only used in test/, then it will be tree-shaken out in the future (#153). However, that still means the hook builds/downloads the asset.

If we'd like to prevent downloading/building the asset, we could consider adding something to the BuildInput/BuildConfig to signal that dev-assets need to be build or not.

However, this also has some downsides:

• Package authors might accidentally not test their non-dev setup because the behavior for a root package is different.
• In pub workspaces there are multiple root packages, and we don't want the cache to be invalidated when running from a different root package.

Thoughts @goderbauer @mosuem @HosseinYousefi